Please note this does not actually allow you to make or upload your OWN meshes, just teaches you how to use ROBLOX's meshes.
This tutorial covers how to use meshes.
- Open ROBLOX Studio.
- Select Insert -> Object -> Part (This will create a new, grey brick in your map).
- In the Explorer Window, select the newly created brick.
- Select Insert -> Object -> SpecialMesh (Your brick should now look like a "head").
- In the Properties window, in the MeshType pulldown bar, choose the option that you wish the brick to look like: