You just make a variable to hold the current weapon selected and change that variable when the weapon changes. In the following code the current weapon variable is "CWeapon". This just uses text but you can make an array (or multiple arrays) to store everything about the weapon (bullets, clips, power level, and so on).
Edit: It uses the up and down arrow keys to change weapons.
dim Weapon$(5)
Weapon$(0)="Knife "
Weapon$(1)="Handgun "
Weapon$(2)="Shotgun "
Weapon$(3)="Sniper Rifle "
Weapon$(4)="Missle Launcher"
Weapon$(5)="Laser "
set text opaque
CWeapon=0
tim=timer()
do
if keystate(200) and timer()>tim+200
if CWeapon>0
dec CWeapon
tim=timer()
endif
endif
if keystate(208) and timer()>tim+200
if CWeapon<5
inc Cweapon
tim=timer()
endif
endif
text 0,0,Weapon$(CWeapon)
text 0,20,"CWeapon = "+str$(CWeapon)
loop
