How do i make a ShopGUI And And BackpackGUI Also an Explosive barrel?

Hello Everybody. I am making a shooter game I have made the guns and everything but I need a way for the players to buy the guns for money they earned from playing the game I need a shop GUI and a leaderboard and also a backpack [not the vanilla Roblox backpack] I need a custom backpack GUI. I need the shop GUI that once the player buys the item it gives it to there backpack GUI where they can later equip and also subtracts the gun price from the leaderboard "Money" say for example you have 200 credits and wanna buy an AR you go press a button that shows a shop you select your gun and buy it for 100 credits it gives you the gun in the backpack GUI and then you select it whenever you want, also I need a crate that needs interaction between the player for 5 seconds and then it explodes (Effect, No Damage) and gives the player a gun randomly. please explain everything if you want to help :)