When you first open GM, you'll get this window. Just click on "Don't Upgrade Now".
Once you've started working on a game, next class just use File>Recent Files... to open what you were working on. If you choose to go File>Open,
make sure you are opening the GMK file (not GB1)
You can work with GM in "Simple Mode" or "Advanced Mode"...
To get into Advanced mode, go to File>...
In Advanced mode, you can have more options for types of resouces (ie fonts, time lines), and you have more options for the properties for sprites and objects.