i am using inverse tan to get theta

I'm glad you've got that sorted out. To answer your other question... you can't turn that direction into a velocity. "Real" gravity wells work like this: the distance of the particle to the well (Pythagoras' theorem) gives the acceleration of the particle.

Wikipedia, or a friendly physics student should be able to fill in the details. You are then stuck with the computational problem: what do you do with the acceleration once you've calculated it? If you divide "time" in your universe into small steps you can use the particle's calculated acceleration to update its velocity. (the change in velocity will be proportional to the acceleration - much as you use velocity to update position) This means, of course, that particles may well have instance variables representing velocity as well as position.