There are several roles that any team should have, so if you want to know what those are, then let me explain it simply so that you can know what operators you should keep on hand at all times:
1 Vanguard: this is a fairly cheap unit which can hold on enemies for some time and tank until the DPS units take them down. Try having 1 Vanguard because it will help you at the very beginning.
1 Sniper: 1 Sniper is enough to deal with the air units, and it can also inflict plenty of DPS. I suggest that you use one in the team and have it prioritize air units.
1 Caster: These are good DPS units which use magic attacks - you will need one when dealing with enemies which have pretty high Armor.
1 Medic: A Medic will help keep the front liners alive, by using all of the heals in their kit. That is a good unit to place behind the front line, because it will help keep enemies at bay for longer.
1 Guard: Depending on the Guard, they are good at defending at some of them are really great at dealing damage too.
Eventually you will be able to have 2 of each class, because that will give you a wider variety of operators to choose from and in case one of them dies and has to revive, you can always use the second one to replace it.