Hi am creating a game engine and one of its parts will be a 3d\2d physics engine. How would I go about this? Also how would I make sure the physics engine and my graphics engine work together and in sync? I would use box2D or bullet but i want to make everything myself.
I recommend looking into "Game Engine Architecture" and you can also google for Entity / Actor Systems or Game Objects.
I encourage you to write your own physic engine and a good start might the presentations by Erin Catto. You can also use Box2D Lite as an orientation. I assume that you have a reasonable mathmaetical background and are familiar vector calculus and dynamics (somewhere on a university level). If you don't have these prerequisites you can still try it, but it will have a very steep learning curve. Another thing to consider is to use Havok (3D) or Box2D (2D). You will learn a lot just from integrating and using these libraries.