There are several factors to consider when building a team. Each character has certain areas they excel in. Some characters are very good at absorbing damage in the front row, while others are better suited to dishing out damage from the relative safety of the back row.
You can use 6 characters in battle at a time, with 3 more characters in the "substitution/reserve" position. When you begin a profile, you will have a limited amount of positions to put your characters in, but as you level up, more of these slots become available to continue to build your team.
There are 4 character "roles" that all cards fit into. The Defensive (Power) type, indicated by a shield on the card, the Magic type, indicated by a staff on the card, the Attack type, indicated by a sword on the card, and the Balanced (All) type, indicated by a star on the card. Generally, the characters with shields will benefit the team more by being in the front row, defending the slightly squishy but more damage-oriented magic and attack types. The All type cards can perform well in both positions, and are a little more balanced stat-wise. Carefully thinking about where you want certain cards in your formation, or "line-up", will allow you to build a powerful, competitive team.
As you acquire more character cards, you will see that certain characters benefit from having other cards on your team. For example, Natsu, when paired with Zeref, receives a 26% increase to his Life stat. This is called a fetter. Examining each of your characters fetters and deciding who will become stronger by pairing them with others will allow you to decide how you want to build your team, and let you know which characters you want to go for.