What problems do you have, and can you solve them? If so, there is your idea. Chances are if you are having a problem, others are too. And if there is something

What problems do you have, and can you solve them? If so, there is your idea. Chances are if you are having a problem, others are too. And if there is something similar out there already, that's good news! It means your market is validated. Put your own spin on it, add some new/different features that are not offered by your competition, and give a go. As for making sure that the time you put in is going to bring in money, well, nobody can be sure of that. However, as I said before, if you are solving a problem there is a good chance others will pay for it. If nothing else, if you solve your own problem then it was probably time well spent regardless.