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.