Research the market. See what other people are doing. If the niche is really obvious there maybe a valid reason why it hasn't been filled
Try and put together a business plan - even if it's only rough
How many staff will you need?
What about marketing costs?
PR?
Technology?
Payment processing?
Speak to an accountant and make sure that you've got it all sorted out properly - it's far too easy to get wrapped up in the "fun" and forget to dot the I's and cross the T's