There's nothing like a good steak, and it's probably a good idea to be able to tell the mesero how you'd like it cooked. It's a good thing I had my Mexican amigos to give me a little help in this department.
If you like your steak well done, then you want it bien cocido.
Like it medium well? That's tres cuartos. Medium is término medio. If you like your steak medium rare, then you could ask for it término medio pero un poco rojo.
For those of you who like it rare, you have a few options - poco cocido, casi crudo, or roja.
There are probably a few other ways to ask for that perfect steak, but hopefully this has given you a good start.