Detect mesh collision

Subject says all, How would i detect mesh collisions? Ive been trying for the past week or 2 but to no avail

Can you explain more! I don´t understand!

in Direct x you have meshes. you know what meshes are right? There the 3d objects and stuff. Im trying to figure out how to keep the player mesh from going through other meshes like walls. something like terrain collision. i doubt anybody here knows how to do that though. im gonna be moving back to C# soon