The part of the grammar page this lesson drills — read it here, then use it.
Group I - the -ам group
Most verbs whose infinitive ends in -ати belong here: drop -ати and add -ам, -аш, -а, -амо, -ате, -ају - the largest and most predictable group, with no stem changes.
Group II - the -им group
Verbs whose infinitive ends in -ити, plus many ending in -ети, belong here: drop -ити/-ети and add -им, -иш, -и, -имо, -ите, -е. The 3rd-person plural ending is -е, not -у - a frequent point of confusion.
Group III - the -ем group
The fully regular, highly productive corner of this group is -овати/-евати verbs: drop -овати/-евати and add -ујем, -ујеш, -ује, -ујемо, -ујете, -ују. Spotting an infinitive ending in -овати tells you the whole conjugation.
Other -ем-group verbs exist too, often with a stem change (see the exceptions below) - those are covered in a later lesson, once you have more vocabulary to practice them with.