Go to your twitch dashboard and find your stream key, when you click the tab it should have something like "live" followed by a bunch of numbers and stuff. Copy that and then, in your stream settings on OBS, paste the stream key in the stream key section. Pick a server that's closest to you, if you don't know it you can either use the ping plugin to find the best server or find another twitch server ping program. After that it's just up to setting your encode settings and all the other stuff and when you start streaming it should go live.