Welcome to the world of improvisation. I'm a relative beginner myself but I've learned this much:
You've made a good start by having the sense to ask for advice from people with more experience. Keep doing this.
Listen (carefully!) to players you like. This is the single most useful thing you can do.
Try to figure out what they're doing and copy it. Play along with the recording if you can.
Learning some theory can help you with the figuring out bit, but it's your musical ear that you really need to develop. The good news is that you can do it. Like everything else in music it's a matter of practice. And what more pleasant way to practice could there be than listening to great players play jazz?
The more I get into this, the more I realise it's all about listening, whether to recordings or to the rest of your band so you gradually learn to "feel" the rhythms and harmonies.
Be patient (I'm presuming you're quite young so you've got more time than most of us here), pat yourslf on the back when it goes well and forgive yourself when it doesn't. Most importantly of all, REMEMBER TO ENJOY YOURSELF. It's surprising how easy it is to forget that last bit.