Welcome to PC! One thing I wish I knew back when I started on PC was isthereanydeal.com, checks a bunch of safe sites to buy games from (most in Steam key form, some just for other stores in general) and can email you when any of them go on sale.
Also, the Augmented Steam browser extension (Chrome/etc) (Firefox) is really nice, sadly no version of it to work in the Steam client itself that I know of, but it adds a ton of QOL to the store.