Cant make my username!

Sooo when i log in, mojang.. and (cant play minecraft, UGH > check out my other problem.) to try to attempt to find a way to play minecraft... When i log in it says the gmail, i created the login with.. am i supposed to put the gmail or was i supposed to put my username, where it was in the creating process.. for an example it said:
e.g BlockBuster
was that supposed to be where i was going to put the username i wanted?