Generally films work with agencies and agents to look for the right actors. You'd need to join an agency, spend a few years doing bit parts here and there, and then be lucky enough to be the one among thousands to get chose. If you're asking these questions, then it's too late to join the cast of this film. Becoming an extra is a bit more realistic, but even then you'd have to live in the right place and know where to look out for information. Sorry buddy, but realistically it's just not going to happen.