To set a global variable to a value, you just use
Do = 5 to GlobalVariable(health)
for example. Although, you might want different ammo per player, in which case you want this code instead:
player.Variable(ammo)
This tutorial shows how to display text on the screen (it shows how to display score, but you can use the same method to display health or anything). Place that text on a separate layer if you’re moving the camera around and then it will stay in place.
There is no pickup system. You need to code this yourself. Something like:
If key g is pressed Delete object pistol
Player is in collision with pistol : Do = 1 to Variable pistol of player
If player.Variable(pistol) = 1 : (code to play animations for shooting, and fire bullets)