I'm pretty sure you need to be American by birth, rather than a citizen. As the previous answer indicates, you need some heavy-weight finnancial backing - running for Pres costs millions of dollars. It helps if you have previous experience, and a high political profile, but the odd-ball candidates appear now and again - Ross Perrot was one, a millionaire businessman with the whim to run, and the cash to back it - he got nowhere of course. The American people aren't stupid ... hang on, thinking about the present incumbent, I may need to revise that thought .....