Skeetendo

A type in 1st and 2nd Gen basically consists of the type's name, its effectiveness against other types, and whether it's physical or special.

In 2nd Gen, the type effectiveness table is located at offset 34d01 and the type names are at 04fee. Types before FIRE are physical, and types after, and inluding, FIRE are special.

The tricky part is that you need to repoint the tables if you are to add a new type, since there's not enough free space around those parts of the rom by default. Repointing means that you locate the pointer of said data and modify it to point to another place on the rom where you'll have more space to work with.

I recommend you sit and watch Miksy91's tutorial where he goes in-depth about repointing data: https://www.youtube.com/watch?v=IRpEwkCITBU