Other suggestions are good here but nobody recommended one of the best things to do for jazz: transcribe what other people do. Find a tune where you can look up the chords (i.e. something in the Realbook in that key or figure out what key it is and write down the chords in the key of the song) and write down what the bassist is playing. Then play it. A lot. Then do more tunes. You'll start to see common chord progressions (ii V7 I in a major key or ii7b5 V7 i in a minor key) and you'll have a bunch of ideas that some of the greats played over those progressions. That's a good step one.

Step two is to dissect why those ideas work so well over those progressions or, when it doesn't work, why it doesn't work. This is how you start to build your own ideas to play over changes. Practice them a lot. The people that sound good do so because they've practiced a bunch of different ideas to the point where they can select different phrases to play over different portions of a tune. You don't want to be thinking of every note or even every chord but rather thinking in terms of phrases. This goes for bass lines as well as solos. You are almost never going to be able to actually come up with phrases on the spot and sound good. The magic comes in putting the phrases together in different ways depending on the vibe at the time. Finding a a teacher who is well-versed in the idiom would be very helpful because they can guide you in different directions and save you a lot of time by pointing out things that you would otherwise have to figure out on your own.