You dont need a tutorial for Marmoset. Once you learn how to make a 3D mesh, UV it, make a diffuse map (texture), Normal (lighting information, gives the illusion of depth/ high detail) and Specular map (tells the engine what is reflective and what is not), then marmoset will make sense. Just task yourself with making a…
If you are going to do game art, you will want to show off your mesh work inside a real time rendering engine. In this case, UDK, Maya or Marmoset Toolbag. For the sake of ease and getting used to the workflow, try going with Marmoset Toolbag. If you have a high rez mesh, you can show it off in the application you made it…