There's something i don't understand about collision:

When i want to detect a collision between a polyeder (obstacle) and a sphere(Player), do i have to check collision between the Sphere and the polyeder(Sep. Axis th. 3d) or between the sphere and every face of the polyeder?

The separating axis theorem will tell you if your objects collide, but not where. Then, if you need the exact point(s) of collision, you'll have to check with every face.

Ok, thanks.

I would build a generic triangle-triangle collision engine. You can then use that engine for just whatever "objects" you check for collision. And you can optimize the engine just ignoring the type of objects that are made of triangles.

you mean even the player as a bunch of triangles?