In what city did it happen?
As a general rule, you must buy the ticket before boarding the tram.
I suggest that you should contact the phone number given on the form (see bottom of the form), and explain yourself and that it happened for the first time. You might get a reduction. When you have a personal travel card that actually covers the ride, but you cannot present the travel card in the event of a ticket check, then will have to show the valid travel card at the counter later and will only pay a small fee.
Travelling without a valid ticket will not be prosecuted after the first time, provided you were cooperative, and you did not cheat. However, when you are travelling frequently, you should make sure to buy a subscription to avoid the hassle of buying tickets for each ride and the risk of forgetting it. Students often get a discount on monthly or yearly subscriptions.