Can anyone recommend a good guide that describes the mechanics of the sorcerer? I'm not looking for a build guide but rather something that explains stuff like spell slots and sorcery points and what your choices are during level up (what learning new spells and forgetting old one does). I had some weird behaviour during level up from lvl 2 to 3 which I'm trying to understand...
You are right, I've been playing some more today (and leveling up once more) and one thing I misunderstood was that I thought a spell slot is fixed to one specific spell. So once I figured out that this wasn't the case, everything made a lot more sense (for example creating spell slots with sorcery points).
You can try reading through the 5e players handbook or googling whatever you aren't understanding followed by 5e. For example "spellcasting 5e" will explain spell slots, ritual spells, learning new spells, and preparing spells in the easiest way they can. 5e means 5th edition and is the version of DND the game is based on.
Some other quick tips, most classes know a lot of spells but can't prepare them all, you can change your prepared spells at any time outside combat. A ritual spell can be cast for free without using a spell slot outside of combat.