New to apple.

I’m a PC guy, but I think it’s important to learn FCP and some other programs which aren’t supported on the PC platform. But I don’t know didilly swat about apple. So I have a few questions, and your advice would be appreciated. First let me say that I really only want a laptop. I can get an external HD with fire wire if needed. Will a G3 work? Is it worth getting a G4? Which OS is best? What should I look for? Is RAM expandable like a PC? And just generally anything you can think of that would be helpful. Thanks!