I was wondering if it is possible to make simple games, such as a tower defense game using game maker?
yes it's possible. Any game is possible in GM (or so I am led to believe) 3D MMORPGs are possible, but not done yet because there's a SH*TLOAD of stuff to do for that kind of game. Basic games like Pong or scrolling shooters are definitely doable.
There's quite a few questions and stuff on the GMC pertaining to tower-defense games, you should browse around there if you're looking to make one.
Yeah, tower defense is something you should be able to do with relative ease. A good first choice, in my opinion. :D
Feel free to ask me whatever you may need to.