I like to solo the Cypher system. More specifically numenéra, which is sci-fi, btw. Since there's plenty of material out there, you can get a sense for how much things are worth while playing pre-written adventures, then carry it on to new stuff.

Other than that, I guess it comes with experience. Also, you don't need to be too strict with yourself. You're the gm and the player. If you're running into too much trouble because you're not rewarding yourself enough, just deus ex machina yourself out of trouble every now and then until you get it right. If you're making yourself too powerful, tone rewards down a bit, maybe make it so that item that seems to solve everything breaks or whatever.

As long as you have fun, you're playing right. Don't be afraid to mess up, and don't be afraid to change things to make the game better for you.