Many of the missions and jobs in Grand Theft Auto Online revolve around killing or getting what you need by way of force. Because of this, you'll need a weapon. It's actually best to have a wide range of weaponry to choose from, but weapons cost money, and many won't be unlocked until you reach relatively high ranks. Of course, if you're going to have a bevy of weapons, you're going to want to add your personal touch to your guns. There's a fair amount of customization you can do to your weaponry, but it all costs money, and there's a good deal of customization that requires you to rank up a bit before it's unlocked.

Some of the customizations you can add to your weapon of choice are functional, while others are purely for artificial. Before you can do any customizations, you should probably head to the closest Ammu-nation store. Here you can browse the currently unlocked weapons, see at which rank new weapons and customizations will unlock, and check out all of the various customizations you can add to each weapon.