To find out what your minecraft username is, just login to your minecraft game launcher on your computer. If you don't remember your login email for the minecraft game, check your email for any emails from mojang.
After logging in to your game launcher, look next to the play button. You should see "Welcome, ___" the ___ is your minecraft username.
All of that is done in the minecraft game launcher.
Next, go to mod.learntomod.com and click the Account gear icon in the middle of the page. Then scroll down type your minecraft username into the blank spot where it should be.