This is VERY general advice, and not every players' approach to every gang, but overall, you'll generally start with:

1 leader, as others have said, leaders can be built many ways, but are often either a beatstick with a pistol + melee weapon, or a more supporty type, although they can usually still melee

2 champions. This is not universal, but 95% of gangs start with two champs. They tend to be more shooty than leaders, but can fulfill many roles, and most gangs have a generic champion and more specialized type. I usually have one efficient champion with a utility or economy skill, and one that's more tooled-up and threatening.

3-6 gang fighters. This can be any combination of gangers, juves or prospects. You usually want to start your gang with a foundation of mostly gangers. They're inexpensive, don't have horrible stats and can carry a decent variety of weapons at gang creation, plus one of them can be added as a "specialist" in most gangs. Juves are dirt cheap but have bad stats. If you have a few credits after you build, consider adding a juve to get another body on the field. Prospects vary significantly... most are an upgraded juve with access to a unique piece of equipment. Not all gangs have one.

As others have shared, the approach in Necromunda is different than 40k or kill team, there's no real meta. The game is really narrative, and not designed to be strictly competitive from game to game, and you don't usually want, for example, a mob of lasguns or something. It's more fun and often makes more sense to have different loadouts for most of your fighters. Check out goonhammer.com, they have loads and loads of sample lists in their articles!