You downloaded your first torrent, let's say it was 5GB in size.
The bytes you receive are called download traffic, and the bytes you send are upload traffic.
Those files you just downloaded came from other users, who had this torrent already.

Just like downloading from others increase your download traffic, when other members download from it increases your upload traffic as well.

This means that, just keeping a bunch of torrents seeding in your client is not enough to ensure your ratio will not drop.
Your upload traffic only increases when someone actively pulls bytes off of your client.

Each torrent has its own ratio, it is a simple calculation: uploaded bytes divided by downloaded bytes.

When you download a torrent and doesn't upload any of it to anyone, your ratio is null.
If you upload the same amount of bytes that you downloaded, your ratio is 1.

In my example where a torrent has 5GB and you download all of it, you can still end up uploading more than that and have a ratio that is greater than 1. It only depends on how popular that torrent is and its demand.

What to look for in order to maintain a good ratio: check how many seeders/leechers a torrent have before downloading. If it has a lot of seeders and very few leechers, this is a very low demand torrent. You're most likely never going to seed enough of it to balance your downloaded bytes.

This is fine though, you can still rely on popular torrents that will make up for it. By seeding popular torrents you can build a ratio surplus (you upload more than you download) which will make up for the torrents where your ratio is below 1.

See the image below for reference:
Some torrents have an empty ratio (no bytes uploaded) while some go way past 1.

NEW TORRENTS: torrents that were uploaded in the last minutes/hour have a high demand. Almost everyone will see it, chances of people downloading it are very high. The quicker you download it, higher are the chances that you will upload to someone else. With time, this demand decreases... People see it less often, the number of seeders increase so much, if anyone tries to download again, they might connect to a different member.

Downloading new torrents is a very easy method of building a ratio surplus, but it is risky: there's no guarantee that you will seed as much as you've downloaded, you can end up with a lower ratio than you had before.

FREELEECH TORRENTS: this is where it's safer to build up ratio. Anything you download is free, it doesn't decrease your ratio. But any bytes you upload for that torrent still counts. This means your ratio can only increase or stay the same, but never go down.

When downloading freeleechs with the intent of recovering your ratio, you must focus on the ones with the least seeders.

There's a sorting option just for that:

If you always download new or freeleech torrents and your ratio doesn't increase, or increases very slowly:

Check your port-forwarding rules in the torrent client to see if it matches the rules in your router. If it matches, use this site to check if it's working: https://www.yougetsignal.com/tools/open-ports/

Type in the port number and press check. If it says "closed" your ability to seed is significantly reduced.

If you get "closed" door status and you are not using a VPN, it is possible that your Internet Service Provider is relying on a carrier-grade NAT, this makes your computer unreachable for other members. You can still upload, but it decreases your chances significantly. It will be extremely difficult to keep a good ratio if you are behind a CG-NAT.

What to do if you are:
VPNs: you can make use of a VPN for torrenting, granted that the VPN provider supports port-forwarding. Here's a list of VPNs that support it:

• ProtonVPN
• Private Internet Access (PIA)
• Windscribe
• PureVPN
• Hide.me

VPN Port-Forwarding is a premium feature so you will not be able to port-forward using a free-tier VPN subscription. Without this, if you are behind a CG-NAT or otherwise cannot get an open port, you are cooked.

Seedbox: Seedboxes are exceptionally good at improving your ratio, because they run 24/7 at very high speeds, once you add a new torrent there the chances of going way above 1 ratio is very high. You can use it for freeleech torrents as well, if you favor torrents that are big in size you can leverage your ratio really fast.