You're running Windows. Not many people try to run relays on Windows, so the process isn't as well documented.
[If you, future reader, are running Linux, then this guide (https://trac.torproject.org/projects/tor/wiki/TorRelayGuide) is shaping up to be amazing. Hopefully it doesn't move. Do not read any more of this post on tor.stackexchange.org and go read the guide on trac.torproject.org. This post is for Windows users only, not you.]
Download the "expert bundle" https://www.torproject.org/download/download.html.en
Extract it.
Find tor.exe. In the same folder as it, create a file named torrc (it must not have a file extension. Confusingly, Windows hides file extensions by default).
In that torrc, you MUST set the following option in order to be any kind of relay.
ORPort 9001
You can use something other than 9001, like 443. 443 is common because it allows more clients to use your relay as a guard, but it requires root/admin privileges to bind to it. 9001 is a common port that doesn't require special privileges. It doesn't really matter what port you use.
You SHOULD set your Nickname and ContactInfo. Note that your contact info will be public.
If you want to be a non-exit you need to set ExitRelay 0 (another common way to disable being an exit is with ExitPolicy reject *:*). If you want to be an exit relay, read https://blog.torproject.org/tips-running-exit-node, especially the reduced exit policy part.
So if you are running a non-exit, you'll have four lines in your torrc. It doesn't matter the order.
You SHOULD also add a Log line so that you can keep tabs on your relay. A good place to put the log file is Tor's data directory. You MAY specify the absolute path for clarity. So I might use
Log notice file C:\Users\pastly\Desktop\tor-win32-0.3.1.9\Data\Tor\notice.log
Read the Tor manual (https://www.torproject.org/docs/tor-manual.html.en) for more options you can set. Generally the default are best. Don't mess with something you don't understand.
You will be interested in RelayBandwidthRate and RelayBandwidthBurst.
If you have issues read Tor's output for clues. Once you have a log file working, read it and share it if you need more help.
Thank you for considering running a relay.
You must stay on top of your relay and keep it up to date.
(Note: I don't run Windows. I'm working from memory.)
My final non-exit torrc would look like
ORPort 9001
ContactInfo pastly@tpo
NickName MyNewRelay
ExitRelay 0
Log notice file C:\Users\pastly\Desktop\tor-win32-0.3.1.9\Data\Tor\notice.log
If I wanted to run an exit relay, it would look like
ORPort 9001
ContactInfo pastly@tpo
NickName MyNewRelay
ExitRelay 1
Log notice file C:\Users\pastly\Desktop\tor-win32-0.3.1.9\Data\Tor\notice.log
ExitPolicy accept *:80
ExitPolicy accept *:443
[... many more ExitPolicy lines based on my desired exit policy ...]
[... the order of ExitPolicy lines does matter: first match wins ...]
ExitPolicy reject *:*
Relays contribute the most when they run 24/7. Please do so if you can. Ideally you'd have Tor start automatically on start up and run in the background. I am unfortunately unable to help with that. But a general Internet search for "how to run console program on startup in background on windows" would be a good place to start.
