一开始工作就可以了吗?还是有什么 要求?是自己办还是公司办?

You gotta ask the company to sponsor for you. If you decide to do it yourself, you also have to get some documents from your company because it has dependency on the company unless you go for NIW or EB1

You can start anytime.. some say you can even start before joinning the company provided it is for the furture position.