Well, first you godda go to the site, make an account, ect.
Once that's finished, you have to go to stream setup. It's there you'll download the streaming application. You can choose OBS or XSplit, but I use OBS.
After that is downloaded, it should ask for your stream key. At the bottom of the stream setup page on Picarto, there's a thing that says Your stream key. Click show key, put the key in to the streaming app, and it should be ready for streaming.
I'm sorry if I've forgotten some things. There should also be videos if you get lost.