Be sure to

a) Define your niche

b) Point out the ways you will be different from others

c) Know who your first customers will be and how will you approach them

Before you start programming/other tech stuff. Simple advice but much easier to preach than practice.

All the best!

This is also something I'd like to dabble in after getting some success with an affiliate site. I have an idea for a niche if you want to bounce some ideas back and forth. Email is in my profile.