Comic books are rife with stories of superheroes who band together to battle evil and defend the innocent. The X-Men in Marvel Comics and the Justice League in DC Comics contain some of the most well-recognized figures in comics, and watching them play off each other as much as the threats they face affords a unique joy that the adventures of solo heroes never quite match. With a little time and effort, you can create your own team of superheroes to protect truth, justice and Mom's apple pie.
Come up with an overarching theme for your team, as well as the name under which they will operate. The big-time comic companies have their own stable of heroes to draw from, but since you're making yours from scratch, you'll need to come up with a reason why they work together and a basic motif to unify them. Maybe they were all created by the same event. Maybe they found themselves drawn together by a common cause or a threat they could never hope to overcome alone. The origin needn't be too developed at this stage--just impart a basic sense of the team's identity and purpose.
Come up with names, identities, powers and personalities for all of the superheroes on your team. This becomes a significant balancing act--granting each character his own sense of self while allowing him to complement the team as a whole. There should be no duplicated powers among the team; each hero should be better at something than any of the other heroes and have at least a few areas in which another hero outshines him. As you flesh out their personalities, decide how it affects the team's dynamic. Do some members bicker with each other? Is there a friendly rivalry going on? Who is the team's natural leader?
Design a look for the superhero team that matches with their overall identity. Sometimes, team members have their own unique costumes, while other teams (like the Fantastic Four) wear more or less identical uniforms. If they wear different outfits, those costumes should be distinct enough to separate them visually (don't match colors or styles). If they wear the same uniform, there should still be enough distinction in the heroes' faces and body types to determine which is which. If you want your team to have a logo, the costumes should probably feature it somewhere.
Devise a headquarters or base of operations for your superhero team. This will be determined in part by their various backgrounds and personalities. If they're Robin Hood-style outlaws, they probably work out of a secret location where no one can find them. If they're public figures, they may be based in a skyscraper in the middle of a city. If one of them is wealthy, he can provide them will all manner of creature comforts, while those without such funds will have to jerry-rig their headquarters with whatever they can find.
Decide what kind of threats your team will face: villains who oppose them, natural disasters they must avert and/or cataclysmic plots they need to thwart. Good villains are an integral part of any superhero story, so make sure yours have lots of personality and constitute a credible threat to your team. Give them a past with your heroes, a sense of overall purpose and some motivation beyond just being evil. Disasters and apocalyptic events should also have a sense of personality. Provide a complex and interesting cause for them, and require the heroes to take multiple steps when resolving them.
Finish your superhero team by providing them with a supporting cast: boyfriends, mothers, friendly cops who might help them out, and so on. Not only do such characters provide good hooks for story ideas, they also contribute to your heroes' human sides--the foibles and vulnerabilities that allow the reader to connect with them. A jealous lover who threatens to reveal someone's secret identity can make as interesting a dilemma as a meteor that might destroy the Earth. Remember that when fleshing out a team's supporting figures.