How to take a picture that is negative on the iPhone 4?

I have already tried putting my phone on negative then taking a picture it didn't work.

Don't do it.
Always take a normal, colour photo at the best settings possible. Then play with a COPY of it.

That way, you:
1) get to have a decent photo
2) have much better control over the process
3) have an original to go back to so you can play some more or play with it in a different way later on