I like to start with something small, that can EASILY be expanded into something larger should I have the motivation to do so.
A great example (as much as we are all sick of it) is the Ghost Shooter tutorial. You could easily remake that as a learning foundation... then easily add to it by making buildings you can enter into, other screens you could visit, or other gameplay mechanics like turning into a round based survival game.
I'd say the top-down shooter is easily the most versitile when it comes to expansion. Platform games come a close second since you can just keep adding levels until you get bored.
I always start with graphics since I am a more art-oriented person. Well, actually I start with a basic idea in my head of how the game might work, then make graphics... then start making the engine, end up asking a bunch of questions along the way, get over-ambitious and stop.
It's hard for me to make something simple and stop... I have to keep adding to it until it becomes something I can no longer control.
Good luck!
~Sol