Well the main thing you will have to do with regards to your UK income is to reconcile it to fit into the US tax year. As such, unfortunately your UK tax returns and P60's will not do much good as they are based on the UK tax year.
If you have monthly pay stubs that show what your income is, then gather those from Jan-Dec, and report your income based on those numbers.
You will also have to report any side or investment income, etc.
I hope this helps! Let me know if you have any further questions.