I know it's a very common question, but I don't see a previous one to be asked, since I'm interested in the basic steps, nothing in particular.

It's the first time I want to license one of my projects, and it turns out, I don't know much about it. Let's say I want to use the MIT License. Do I just take the definition and paste it into my files? Does it need to be in ALL the files, or just one? or?

I don't want any fancy rights so MIT has been recommended to me, do you suggest something else? I'm looking for impartial opinions. Thanks.

What other aspects must I take into account? It's PHP, btw.