[matkiros]

Here's what I personally did when I had an idea for a startup:

1. Decide where it's going to be most useful & get a lot of traction: desktop, web, or mobile. I had no second thoughts about going mobile since the problem I was trying to solve was mobile-related.

2. Design the screens. It doesn't have to be pretty, and don't take too much time doing it because whatever you write down will 1: most likely change during the dev process, and 2: most likely change upon testing it with your users and receiving feedback. Just use it as a guide. Then, code.

3. I don't think investors would be willing to give you some seed fund for a product that doesn't even exist yet. So definitely get started with at least a working prototype, would be great if you have some data to back that up (what did your users say, how many downloads have you had, how much money did you make from it, etc etc).