Have you actually read the rulebook yet? You can download the rulebook and supplements from here:
http://www.games-workshop.com/gws/content/article.jsp?catId=cat480009a&categoryId=1100011§ion=&aId=5300010
Also take a look at my Yakromunda site for managing your gangs and campaigns, makes it a lot easier to keep track of them over the course of the campaign, even for creation.
For the gang make up itself, you learn over time what combination you prefer, but you have specific requirements for gang make up detailed in the rulebook, like 1 Leader, 0-2 heavies, at most half the gang can be juves etc.
For a decent starter gang that you can make sure you'll get enough income from territories, start with a Leader, Heavy, 5-6 Gangers and some Juves. Don't worry about equipping your Heavy/Gangers with backup weapons at the start, those can come later. Depending on how much credits you spend, also consider taking a 2nd Heavy at the start but you don't need to buy him a Heavy weapon, fit him with a Special or just a Basic weapon.