You want to base how you build off of combos/synergy. Start off with you main character and see what moves it has and what they do. Most attacks have an effect like High Float Low Float etc. this is what you use to combo with. Starting with your main character make a chain of these effects and you will have a combo, that is the basic of building with smart offensive synergy. To build a overall strong team you want multiple start points for your combo, say if my main does Repulse and it starts a combo you might want to have another team mate that can start that one or if one can start a combo with another effect thats even better, since when character faint you might lose your ideal combo chain.
When doing formations you want to base it off of defensive needs and offensive order. You usually put you healers and highest damagers in the back row. Tanks and some times your main based of how you boost them can go in front. ex. With team 7 (Naruto, Sasuke/Kakashi, Sakura) you want Naruto in the front and the other towards the back from a defensive standpoint since Naruto can take hits and uses Multi-Shadow clone at 2 star which is even better at 3 star to help him out. Offensively how you orient them effect what order they attack in, you usually want your healer or support person going last and your other members going first.
All in all its about thinking what does this member add to my team and how can I link all these people together. Pay attention to the available matches *on I believe its called. You want all your members to be able to combo off each other to have a viable PvP team or just in general