The following are notes and musings from my years of bass playing. They are really just observations based on experience of playing in two different sorts of environments, the church and the rock band. If they help anyone else, all well and good.
It may sound obvious, but ask yourself why you want to play bass in the first place. If the thought of playing with other people puts you off then you might be better choosing a different instrument, as the bass is predominantly intended for play in a group setting, be that a three-piece power trio or an orchestra.
Some people would say that you need to really know how to play your instrument before you even think of joining a group. I disagree. I think it is playing with other people that sharpens most musicians up, and until they do that they are doomed never to progress beyond a basic level of competence. In particular, most people's sense of rhythm and timing is definitely improved by learning to "lock in" with others.
Having said that, it is a bit optimistic if you have never played an instrument in your life to buy one and join a group the same day. If for example you didn't know that bass guitar isn't played with a violin bow or thought that you plugged the instrument directly into the electric socket in the wall to get the sound amplified, then you should spend some time learning the basics first before venturing out with others. Most people are tolerant of simplicity or basic playing in bassists, but your fellow musicians will get tired of carrying you if you are perpetually playing wrong notes or failing to keep in time.
Really, no musician can afford to be out of time, period. However, because people look to the bass and drums to provide the absolute rhythmic foundation of music in a band, then both these instruments have to be tight. Solos by treble instruments may be allowed to wander in and out of the basic rhythm and even to be slightly off beat, but this is usually highly undesirable for the bass. Bass is a time-keeping instrument that with the drums lays down the rhythm.
Rhythm is such a deceptively simple concept yet such a vast field that a short essay like this cannot do it justice. Suffice it to say that whereas in a group the guitars and keyboards may be thinking primarily of their chords or melodies, the bass should primarily be thinking of the rhythm to drive a particular song or piece of music. It should also be stressed that for a lot of music, the rhythm is fairly straightforward and will not benefit from a frustrated bassist trying to make it more complex than it needs to be. This applies in particular to music in settings such as formal occasions or church, where all musicians are usually trying not to draw too much attention to themselves as this would be inappropriate. On the other hand if you are playing prog rock or free jazz then by all means feel free to experiment, as long as you are aware that this sort of thing is very much "musicians' music" and that the majority of a potential audience may not enjoy your artful meanderings or ideas. (The same applies just as much to self-indulgent guitar, sax or keyboard solos!).
When playing with a drummer, there are two extremes which I have found. One is the sort of bland fare where the bassist locks in so much to the drummer that every note he plays falls exactly on the bass pedal, a state of affairs usually made worse with this sort of music as the drummer himself is often playing a fairly predictable rhythm. The other is a ragged and unsatisfying state of affairs where the bassist is actually not listening to the drummer at all and playing a completely different rhythm, which can at times sound like the rhythm section is playing two separate pieces of music simultaneously. This is really where practising with a drummer or drum machine can help you develop that important feel whereby you can make the basslines sound interesting while keeping a tight rhythm. Bass fills can add a little interest as long as you don't overdo them, just as drum fills are good provided they don't make the entire song sound like a drum solo.
This brings us onto how the bassist knows which notes to play. Basically the "knowledge" is comprised of two parts: (a) knowing the song and (b) knowing enough music theory and scales to fit the right notes. If words like "theory" and "scales" sound offputting or even frightening, they should not be. If you can count, you can learn music theory, and scales are just like learning the basic multiplication tables. Unlike some guitarists, who seem to feel the need to learn every possible scale combination known to man from the last thousand years, bassists can actually make do with a few scales, the most important being the major, the minor and the blues scale. The blues scale has such a distinctive feel that it tends to be used in rock, blues and jazz but not in other, perhaps more solemn or melodic music.
One aspect of playing electric instruments that is often neglected is that of tone. These days active circuitry and graphic equalisers on amplifiers have given us such a range of options that there really is no reason not to experiment, although it can be daunting to remember your preferred settings if you are spoilt for choice in this way.
It is quite hard to replicate the sound of the double-bass, and to be honest if you really want to sound like one then you'll probably have to either buy the real thing or a MIDI effects unit which will track your notes and convert your signal to sound like it. Once you realise that, paradoxically you may feel freer to experiment. There seem to be no hard and fast rules about different tones appropriate to certain settings: however, it is also a fact that whereas treble-heavy bass cuts through more audibly, the bass end of the tonal range seems to resonate and carry further. Really you will have to play around to find what is best in your group situation.
It seems to be a rule of music that the more instruments are involved, the more tightly defined the parts for each instrument are, and often the less freedom each has to stray outside those boundaries. If you play in a guitar, bass and drums three-piece, for example, you will have a lot of room as the bassist to play your own parts, provided that they do not clash with the guitar. In a group with keyboards, horn section and maybe more than one guitar, you may find that you have less room to manouvre, and that riff-style playing higher up on the neck clashes with the other instruments. In a group context, however, keyboard players need to be aware that in most cases, the bass parts are much better left to the bassist to concentrate on: fancy left-hand playing is good for jazz piano but often clashes with the bass in other settings. In older jazz, pianists used to play chords with the left hand in the octave below middle C, leaving the bass to play the low runs. I still think this is a sound principle in many places, and in any case the use of synths and MIDI controls these days often encourage keyboard players to keep one hand to free to press the necessary buttons.
Perhaps it is harder for bassists to develop a style of their own, although the same could be said of many instruments, especially those not traditionally regarded as solo instruments. To a large degree the development of your own style will be influenced by the sort of music that you want to play. If you are doing country, for example, the simple basslines played in that genre will mean you are often playing roots and fifths on the first and third beat and restraining yourself. That is not a criticism of country: it is just how that style sounds, and anyone trying to be over clever while playing such material on the bass will end up just sounding "wrong". Even those styles which at first hearing sound like they offer much more scope to the player (eg slap bass, heavy metal) can end up forcing the bassist into a certain way of playing. For this reason it is often a good idea to experiment with different styles of music and to play the various examples thereof from exercise books.
I'm always a bit ambivalent about bass solos. While they can be fun, they can sound dire in the wrong place, or else be subject to the law of diminishing returns, ie they're worth less the more they're played. Nevertheless it can make an interesting change from hearing yet another guitar or sax solo, and the discipline that helps form bass solos also helps you in other areas such as walking basslines. It should be realised that most people however do not want to hear a bass solo in every song. Even outstanding and renowned players such as Jack Bruce or Flea achieved their fame more by making interesting basslines than by thrusting themselves into the spotlight with endless solos. The law of music seems to dictate that as far as rhythm sections go, the solid musicians tend to get the gig over the flashy ones.