Different torrent clients have options for that. Most will seed indefinitely by default, but some you can set to seed just for a specific amount of time or until you upload a certain percentage of what you download.

If I sign up for a torrenting site like torrent galaxy or something

That’s a public tracker/index. You don’t need to sign up at all to connect to a public tracker swarm.

and leave my computer running then will other people get seeded?

Yep. You can set bandwidth limits if your upload and/or download internet package is bad so it doesn’t interfere too much.

Or do I have to upload a file or can I just have it finished?

Hit and Run is considered bad manners and selfish behaviour. You can get away with it on public trackers, but private ones typically ban you for that. It’s called leeching. It’ll be based on your total upload/download stats, or your ratio in other words. Aim for 1:1 minimum. I’m pretty sure all torrent clients support displaying ratio even if it’s not enabled by default in some, possibly.

Are you asking about the basics of how tormenting torrenting works and how torrents are distributed? Nothing wrong with that, I just want to make sure I don’t write a wall of text, explaining something you already know.

Edit: Spelling. Ducking autocorrect.

The account doesn’t matter. What you need to do is leave the torrent program on your computer running. And the files you want to seed need to be in the list of that program.

Also make sure you know what you’re doing. You might be pirating and that might come with legal consequences. So read about the law and use protection.

Be safe. Use a good VPN for pirated content and don’t trust that what you download is what you expect it to be.

If you are in the US, or any other country that has strong copyright enforcement, using a VPN is an absolute necessity. ISP’s usually have some form of 3 strikes policy for being caught downloading copyrighted material. As in, 3 strikes and your disconnected.