I don't know how much you know about engines already, so I'll assume it's very little for the sake of this post.

You want to start with a four stroke. That said, you want a very basic four stroke, as the mechanical bits are all surprisingly similar. The camshaft may be in a different place, the quantity of valves and cylinders may vary, the ignition system may be different, but they all come down to the same four strokes and nearly the same method of handling the fuel/air mixture. So-

Get your hands on a busted old lawnmower. Get a service manual for whichever engine it has, tear into it with abandon reducing it to as many little pieces as possible, reassemble according to the manual, and make it run. You now have a solid foundation of engine innards, and a working lawnmower.

Repeat with a busted old four stroke motorcycle, this time expanding out into the transmission and suspension and all that. Get the factory manual. Go to tech days, read up on things, take things apart, ask questions here, find a forum for whichever particular bike you have. When you're done, you'll know quite a lot about general motorcycle repair, quite a lot about that particular bike, and hopefully have a working motorcycle.

Don't worry about breaking things, you will and that's the point. Nine months is pretty short time, but you'll still gather quite a bit of useful information by then.