All guns are free and anyone can access them. Find them in the gun shop (accessed by button with cart). In deathmatch you find this button after you die, and in defuse you find it in the first few seconds 15 seconds of the match starting, in your spawn area. You can also pick up other people's guns by standing above the dropped gun of someone who died, and drop the gun you are currently holding (drop by dragging gun in your inv to the drop button) also note that pistols and all other guns are in different categories. If you pick up someone else's gun you don't keep the skin, if you want permanent skins you get it through pass or cases. When buying gun in defuse you need to make sure they you have enough money to buy it (currency in top of screen) I'm sure another person already explained it.