To start a gang, you click on the Gang Button (mobster face) which exists right of the Crime button (Gun).
The one who starts a gang must be at least level 3 and will also need 25,000 $ to pay from his own account to start the gang. While creating the gang, you get to pick a name and a short name while for those who want there are extra options to insert gang's homepage, gang's description and gang's irc channel. This info can be changed later except the name or the short name, so choose carefully.
Once you have started a gang, one of the best places to get new members for your gang is the Players Joined today page.