Drop the time limit. The added pressure could easily lead you to walk away from lucrative projects that are not "fast" enough.
Find something people want, build it, figure out a way to monetize, and then scale up with increases in demand.
Alternatively, make flashy interesting things that investors are willing to throw money at (Your age helps you here), pay yourself well while hiring solid talent, and have a fast exit strategy like aqui-hire. Wash, rinse, repeat.
Either way, a solid accountant, as well as a solid personal grasp of accounting, will be a lifesaver.
I say easy, but coming up with a good idea is easier said than done. Find a market that's missing something. Something people will use. Do some research, plan your project and work like hell. Startups aren't easy. It takes a lot of dedication..