First step is to validate the problem. Ask 20 people in your target market about their biggest problems. If they bring up the problem early in the conversation, you know you got a winner. If you gotta pull it out of them, you'll have trouble getting them to use whatever solution you come up with.

Then you brainstorm on a solution based on the notes from the interviews.

Then you interview another 20 people and pitch the idea to them. At least half of them should seem excited about it. If not, you gotta either rethink the solution or the problem.

And if you can't find 40 people to interview, that's another problem altogether.

Good luck!

Would you say that by asking ppl to fill in a simple Monkey Survey be sufficient, in case you're looking for a World wide audience?

You won't get enough data if you ask them to fill out a survey. If you try a single interview you'll see why. You'll end up with pages of notes and venture into problems you had no idea about. If you had asked them to fill out a survey instead, you would've ended up with just a fraction of the information.

And world wide or not doesn't matter. I did all my interviews over Skype.

Awesome! Just a question - how did you go about finding interested candidates who wanted to participate and take a Skype call?

Reach out to your network or email your friends to put you in contact with potential users. It should be moderately easy since you're just asking for a free Skype call, you're not asking for money yet.

If you're having problem finding people to get on a free Skype call then it's a big red flag. You'll need to reconsider your positioning or your market. Because eventually you'll (presumably) be asking for money and that's even harder.