Write down a basic description of each hero on your team. Include powers, personality, costume and reasons for fighting crime. The descriptions don’t need to be detailed, just enough to give you a sense of what each hero is all about. Make sure there’s no overlap -- you don’t want heroes with similar powers or themes--and ensure that every hero plays a viable and distinct role on the team.
Flesh out the background of each hero. Develop their personality in as much detail as you can, as well as drawing pictures of them in costume. (If you don’t have a knack for drawing, you can use online resources such as Hero Machine to create costume images for you.) Be sure to include details about how they gained their superpowers and the ways in which their superheroic exploits affect other aspects of their lives. The more information you can provide, the more readily you can come up with good stories for your team.
Determine how your team got together: the circumstances which forged them into a single unit. Particulars vary widely. In the case of the Avengers and the Justice League, they joined together to face a threat that no single member could handle alone, while the X-Men were assembled by a single leader and mentor, Professor X. Detail your team's origins as much as you can, then write it down and add it to your hero descriptions.
Develop some villains and threats for your team to handle. Every hero has his share of bad guys to face, and hero teams even more so. Flesh yours out by following steps one and two for each villain, then determine how they first came into conflict with your team. That will give you good fodder for future stories featuring that villain.
Look for interesting stories within the team itself. With super powers come big egos, along with personal demons, insecurities and basic personality conflicts. While some members of your team may be great friends, others may chafe at the antics of their teammates or disagree about the best path to take in a crisis. That friction should never break into open hatred--they’re still on the same side--but it makes your team interesting in ways that have nothing to do with their powers.
Come up with a catchy name for your team--something that sums them up--along with a logo that draws the eye on the comic book page.