Gas Caps for Dummies

[TallGMC]

Hi All,
Newbie here and love this site! I dont have much experience with mechanics but could use some help. I ordered a new gas cap and it doesnt really fit. It just sits on the filler neck. It will lock but some days it doesnt seal because I can lift the edge and wont hear the vapor hiss. I have been reading about how there are vented and non vented caps/tanks. How do I know what I have and what I need. I have attached a pic for you to see the current cap. Help me out! Thanks!!!