ITeam wars are easy enough:
You choose one team of defender for your island and you have 24H to strenghten them.
Then , when the defense phase is done you have 24H during wich you attack enemy islands. You usually get 5 attacks per phase (10 on rare events). During that phase you get 1 team coin by monster who survives the attack of an enemy island (15 total if you do your 5 attacks, you can raise that total to 30 on events)
In these wars, don't bother attacking monsters that are stronger than yours. You want coins so you want to match strenght. Also match types your monsters will be destroyed if you have commons at level 100 and you attack legendarys at level 100.
Also monsters at same levels will get owned if the enemy is filled with stronger runes, so keep an eye on that when you choose your attacks.
If none of your monsters are strong enough, tell it on the chat or e-mail it. Dont let teamates think you are not participating.