I play Leona Support.
Wave clear the minions if you can first. Use your W (your defensive buff) it will help clear.
If they are close to tower you can also use your E if you are not low and they are not really strong. It will root them and it they attack you they may drop into tower aggro range.
Save your stun to escape or stun them after they draw tower aggro.
It really does depend on levels though. If you are both low levels you can probably trade without taking too much damage. First though, just simply wave clear with W, but have you E ready if they auto you within tower aggro range.
If they make a serious mistake stun them under tower and drop ultimate. In general, the ADC should respect a Leona under tower.
Sometimes you can also just walk up to them and stun them while they auto the tower. Just harassing them a little draws their attention and damage off the tower.
Edit: It is really important that you thin out the minion wave before touching the ADC. If the tower is below half health and the ADC can kill it fast and they are strong (early mid to mid game) give up the tower.