[Request] What should I study first for making my mod?

Hello, I'm now interested in making mod for BG:EE, especially classes/kits revisions/rebalancing, but I don't know what I should first learn for it.

I've heard several modding tools such as DLTCEP and Near Infinity... Could someone give me hints how to make such mod by using the tools?

Some reference would be welcome, too. :)