5th dnd – How can I gain control of the medium armor as a thief?

Certainly

See the Player's Manual p. 164.

Multicasting for a single level of cleric, druid, fighter, paladin or ranger will allow you to master the average armor.

You can also acquire this skill by getting a level in some subclasses of other classes (such as a Hexblade Warlock from Xanathar's All-Around Guide).