Noob Issue

[2MuchHustle]

Jus started playing on LVP(great btw) and i cant figure out how to get purchase weapons.I go to Ammunation and stand in the red marked and the menu pops up but it wont let me select anything in it.Just lets me scroll up and down between weapon types(pistol,....)

someone please help me i am tired of getting killed because of my inability to fight back