The shop has 6 main categories: Weapons, Armor, Equipment, Skins, Crates and Credits.
More explaining about it below
Once you gained a blueprint for a weapon, you can buy it in the shop with credits. You can gain credits through Credit Boxes, by killing people and having double blueprints. I mean that if you already got a blueprint and gain that same one again, you will get credits, instead of getting nothing.
Also armor has to be unlocked by blueprints, and then credits. Armor grants you mostly big advantages like extra health, or walk/jump boosts.
Skins are only available for purchase with ROBUX, and give you only cosmetic advantages to the others.
Here you can buy crates with ROBUX.
You can also buy credits using ROBUX.