Because I don't have much else to do, i've decided to write a book with Geralt in it, containing one of his many contracts.
Please refrain from calling me a twit. How would all of this work? Am I allowed to post it on the internet? After all, other people own the rights to the witcher universe.

Geralt of Rivia (and the Witcher universe) is copyright. You need to contact the author's agents. Don't get your hopes up.

Ahem, you do realise there is already a book series centering around Geralt & co.? Just saying, it's a recommended read ;)
There is some fanfiction out there, and I don't see any problems regarding copyright infringement, as long as you keep it on a non-profit level (mind you, most of the fanfiction is of a quality that I highly doubt people would actually pay money for it).
So, go knock yourself out :)

So you're basically writing fan-fiction. It's a bit of a grey area but AFAIK, Andrzej Sapkowski (the creator of Geralt, author of the novels) does not oppose to it as long as you don't try to sell it or publish it as your own (i.e. original) work. Just be clear that it is fan-fiction and you should be fine. You're not gonna get sued or anything if you don't make any money from it. There's actually plenty of stories from The Witcher universe around the web.

Be aware that the books are considered canon by Sapkowski and many fans (including me). While the games do fit the canon VERY WELL, they're a bit of a different universe. It's more like the GAMES are centered around the characters from the BOOKS. Just saying ;P

Yeah, I am aware of this, I just wasn't sure whether this is common knowledge at this point and didn't want to appear condescending ;)
Kudos for being a book reader, btw.

I can't really see how would that be 'condescending'. You'd just be stating facts, but whatever. Let's not derail the thread anymore.

So, I can make it and post it on the internet as long as it's free. That's fine by me.

Yes, pretty much. Many people put a sort of disclaimer at the beginning just to be on the safe side (something like - all Witcher characters are not mine and belong to the author of the books or the game developers).
Other than that, you should be just fine.

You know there are already like 7 books right ? That's what the games are based on. I highly recommend you read them, they are awesome.