Achievements in TF2 are the same as STEAM Achievements. In order to aquire a weapon/clothing you must complete a class' milestone achievement, such as getting ten kills as pyro (not a actual achievement). This will grant you a weapon for that class. In total there are 3 achievement weapons that you can aquire for each class. Other achievement items can be aquired from killing bosses in Man vs Machine. To get cosmetics you must complete achievements such as the Ghastly Gibus Grab (Dominate a player wearing the Ghastly Gibus to earn your own.) You can View how to get all achievement items Here. NOTE: THIS IS NOT A STEAM AFFILIATED WEBSITE USE AT YOUR OWN RISK click here

In multiplayer all you have to do is play! You can be in Man vs machine, or casual play. You earn a weapon every 3-5 Games. This is very easy and is fun too! I earn a lot of my items from it!