So, you want to start streaming? Cool! It can be intimidating, but that's why I'm here (to intimidate you - no wait, that's not what I meant!).
You'll need some sort of streaming software. For streaming from certain game consoles, from an iPad, or some other platforms, there may be a native app for this purpose; for most people, I recommend OBS Studio, or one of its relatives.
Whatever you pick, there's probably an automatic setup option that will do most of the work for you. Let it pick a bandwidth and resolution for you if possible.
A webcam is completely optional for many types of streams. If you don't have one, don't worry too much about it; if you have a "bad" webcam, that's also fine. The most iconic streaming webcams are the Logitech C920 and C922, which usually cost about a hundred dollars; they'll serve you well for a face cam, or for any sort of work camera (eg for traditional artists).
Got a basic headset? That'll work magnificently. Got a bog-standard laptop mike? Even that will do for a start. You don't have to blow big bucks on a professional microphone! Pretty much anything will do.
Log in at https://twitch.tv and pick your user name. It can be changed later, but there are consequences to changing, so try to pick well if you can.
Go to https://www.twitch.tv/dashboard/settings and access your "Stream Key". You'll need to give this to your streaming software.
Go to https://dashboard.twitch.tv/settings/stream and enable "Store past broadcasts" - this will allow people who missed your streams to go back and see what happened, and also allows YOU, the broadcaster, to easily see how your streams are looking and sounding.
You may need to enable two-factor authentication on your account too.
Have a very basic stream setup, and just hit that "Start Streaming" button! The fun has begun, and it won't end any time soon :)