Evokers can be found in Woodland Mansions, which are a rare structure that spawn in Deep Forest biomes, as well as sometimes appearing during Pillager Raids, which you can find a guide on how to trigger here: Solution for We're being attacked! in Minecraft
All you need to do is kill an Evoker. The mob is quite dangerous, as it uses magic to summon various attacks which deal quite a bit of damage, so make sure you are geared up with at least a full set of iron armour. When killed, it will also drop a Totem of Undying, which can be very useful.