There are some who call me...Tim achievement in Borderlands

There are class mods, which amplify the abilities of a certain character, which you will eventually find from drops and some weapon/item crates.

Class mods do have class requirements, so you must find a class mod which applies to whatever character you are playing as. (E.G. You cannot equip a Soldier class mod to Mordecai)

Simply equip the first class mod you find for your character and the achievement will unlock.

** SPOILER **
You should get your first class mod after killing Sledge. (Credit to Ruthless Remix)