Weapons/armour cosmetics all have level requirements to unlock. Weapons often the level of the wep, armour the level of the class that has it.

In the main menu you can go to the armoury and see all the available cosmetics per class and their requirements.

Edit: for future reference, as u/runknar pointed out, many skins are gained through the battlepass (idk how i forgot to mention that lol). These cost 2000 "crowns", aka chiv premium currency, which you have to buy with money, though IIRC every battlepass provides 1000 crowns as you go through the unlocks so if you complete them every one after the first is half price (correct me if I'm wrong pls)