Twitch is a live broadcasting service where they'll push the video stream to those who are interested in watching you, but it's pretty much a DIY deal... In short, you'll need the following...
* A stable broadband internet connection with a minimum upload speed of 1.5 Mbps
* A computer that has at least 8 GB's of RAM & a decent processor
* The desired game you wish to broadcast (for PC) OR a video capture card (for consoles)
* Broadcast software (XSplit, OBS, GameShow, ect.)
* (Optional, but highly recommended) a webcam
* (Optional, but highly recommended) a microphone / headset
PS4 & XBox One consoles will have Twitch support built-in, but the interface there is pretty basic. PC (& consoles through a video capture card) gives you more control over your look, but it takes time to design your overlays.
You can visit
https://help.twitch.tv/customer/en/portal/topics/925393-beginner-broadcasting/articles for the basics &
https://stream.twitch.tv/ for recommended settings, but here's some things I've discovered through my own experience.
* You should broadcast AT LEAST 720p @ 25 (for PAL) or 30 (for NTSC) FPS (whatever the norm for you) -- This practically the unspoken baseline for Twitch, even if the source game runs at a lower resolution (like 240p or 480p). If you have the processing capabilities & the bandwidth, you could go up to 1080p @ 50 / 60 FPS.
* Unless you're sharing your internet connection with other broadcasters, you should set your broadcast speed no higher than 80% of your upload speed to maintain a stable broadcast (with minimal frame drops). As noted above, if your upload speed is 1.5 Mbps (~1500 Kbps), you should limit your broadcast speed to 1.2 Mbps (~1200 Kbps).
If your upload speed is around 5 Mbps, the max for your broadcast will be 4 Mbps (4000 Kbps)... However, you could go lower (around 3 Mbps / 3000 Kbps) at this point.
Twitch recommends going no higher than 6 Mbps (6000 Kbps) as it starts to become overkill. I do have to mention that unless you're a Twitch Partner or at the very least an Affiliate, you do have to take your broadcast bitrate into consideration. Setting it too higher may throw some users into "rebuffer hell" (where the viewer's connection isn't strong enough to download all the necessary data in a timely fashion, causing their stream to pause very few seconds). HOWEVER, setting the bitrate too low will cause quality issues... namely pixelation (where things look blocky or blurry).
Twitch provides transcoding services (where Twitch will broadcast a stream at various qualities lower than what the broadcast is using) to Partners first, then Affiliates seconds, then finally standard users with a reasonable viewer base for a broadcast (this can be at low as 20 viewers) as resources permit. For newbies, this is something you probably won't see for your stream until you have small viewer base (minimum average of 3 viewers per broadcast) to qualify for Partnership Affiliation.
* Unless you're going straight from your console (PS4 or XBox One), spend a few hours to develop your layouts. Minimalistic layouts are fairly decent, but you'll need to have some personal watermarks & some green-screening to pull it off. Otherwise, try to do some framing to avoid hiding important stuff in the game.
Hope this helps!