One way to begin with gs is the manuel.
first room, first door, first lights...ok it is not the "modern" way because the most levels are models today, but it is a beginning.
maybe,if you have same exp with gamestduio, make a template-shooter or a short game
but byw... if will you work alone then you have to lern alot. C-script, too.
How long it takes, until you can create your first game, i can't say. If you learn fast, you can begin earlier.
ps: You can find a lot of tuts. in the AUM and AU.
