Get an idea from problems in your own life. If you don't have problems that are original enough, become a more original person. Don't build products that are solutions in search of a problem.
Launch early and multiple times. Launch to famous startups websites (like Product Hunt, Hacker News, The Next Web), mainstream websites (like Reddit) and mainstream press (like Forbes).
But more importantly, find where your specific audience hangs out on the internet (or in real life) and launch there. Launch in a friendly way, that means "here's something I made that might be useful for you", instead of acting like you're some big giant new startup coming to change the world.
Grow organically. A great product that people really need which is better than the rest will pull people in. You don't need ads for that. Don't hire people if there's no revenue yet. Don't hire many people if there's revenue either. Stay lean and fast. Do things yourself.
Monetize by asking users for money. Don't sell their data. Don't put ads everywhere. Don't dilute your product. Be honest that you need money to build the product they love and they'll be fine paying for it.