I'm an improvisor of long standing. One thing you might want to try is starting with the melody, and then making small incursions outside of it until you get comfortable enough to be more adventurous. This gives you a familiar point of reference that you can always return to if you end up too far out on a limb. It also helps plant the harmonic structure in your head, so that you have the intuitive kjnowledge of it to go along with the "book learnin'" that Rick suggests. Have fun -- improvising is one of life's great pleasures.