check the I-130 instructions what the documents are required. K1 visa is not recommended since you're married which also needs the further process in USA as well. I don't thing there will be big time different between US citizen married Vs K1 process.

don't need a lawyer, follow the i-130 instructions, he will do the same what you do.
need a proof of marriage ( i got mine from the metropolitan office / NagarPalika in English with the pictures of both), police report and other documents as requested. It's not hard if you follow the i -130 instruction.