how to add weapon shop gui to the game ?

hey guys i want to add a weapon shop to my fps game and i don't know how to do it ? i have used many scripts and plugins from the web but they were junk can anyone give me a good script and say where should i put the script pls ? and one more thing i've added in game money but don't know how to use that money for buying from my game's shop (players start with 800$) thank you