The alternator doubles as a tensioner, and it's on top of the engine. You will need some metric-sized sockets to loose the two screws that's in the traveling direction of the car that holds the alternator, circled with red in pictures below (only two turns or so) and then there is a third vertical screw which is used to adjust the tension, circled in green. Turn it one half to one turn in the direction that makes the belt tighten (it should be clockwise).
Too much tension puts unnecessary strain on bearings and the belt, but a turn on the tensionin adjustment screw will not cause much problems.
Tighten the two screws holding the alternator again.
Since the alternator is on top of the engine this is on a level where you don't even have to worry about your knuckles.
And while you are at it - check the belt for damage and cracks. If it shows some small cracks it may be a good idea to plan to replace it.