HELP! A/C problem

I have a 2000 GMC 1500 Z71 Sierra 5.3 liter truck (LT)

It's the first time it went out. It still blows air but not the cold air. Do I need to add freon or do I need to do something more complicated? If I just need to add freon, where do i add the freon? If I need to do something more complicated... Can you please send pics & pin point where & what I need to fix &/or replace!

Thanks.