ROM Hack Starting ROM hacking

[WorldSeven]

Hey, there.

I wanted to ask some "beginner's questions". I'm new to Rom hacking and basically don't know anything about it.
I'd really like to do some Fire Emblem Fates Hacking but except from very basic Hexadecimal knowledge...(very very basic) I don't know how things are done.

So I wanted to know how to start? Or is there any beginner's thread somewhere around here?
Or how do I generally modify things in a ROM? (I know with an hex editor...)
The question I'd like to know most would be probably how to find out which thing in a hexadecimal file stands for what? (for that is just a bunch of numbers and letters for me...)

Thanks in advance. Any answers are appreciated.