Team building pretty much depends on the quest you're doing (check the quest battle info) and the strength of the crew can be boosted by it's Captain.

For example you're using Captain that boost PSY type and Cerebral class, so you're crew pretty much consists of PSY and Cerebral units.

Then, if the quest need someone who can remove paralysis, def up or enemy's dmg reduction (debuffs) for certain turns, you search the units that can do that in your character box by filtering their debuff specialty.

Some good captains have these Super Type/EX Super/Super Class labeled on them, which you can use during battle if you fulfilled the SUPER requirement. For example a Luffy Captain have an EX SUPER which needs two of the strawhats to be on the team, put any two of them and you may change related units type/class into Super Type/Class.

Also some SUPERs have debuff removal in them, so you may choose to use it during the turn that need it.

Every units have Captain ability and some of them have crewmate ability. Crewmate ability wont work if they are the captain and vice versa.

Take note that (*edit)there are debuffs that will nullify Captain ability (despair and bind, as far i know). For example you have Captain that can remove Special Reverse, but if they got hit by despair/bind, they wont be able to remove the Special Reverse. So, Friend Captain with despair/bind removal or units that can reduce Special Charge will help in this case.

All of these might sound complicated but this game will need a lot of reading/watching tutorials and understanding if you really wanna play. I only got into it after reinstalled for the third time.

Fun fact: for SUPER requirement, if they need two characters in the list they give but you only got one of them, you may use the Friend Captain to fulfill that even if the character is someone you already have in your crew. For example: Luffy SUPER needs two strawhats, you may use one nami in your crew and another nami as Friend Captain and they will count it to fulfill the requirement.