To equip a weapon, highlight it in your toolbelt with [Arrows] and press [Frob]

To unequip a weapon, highlight an equiped weapon or an empty toolbelt slot and press [Frob].

When you equip a ranged weapon, an ammo counter will appear in the lower left section of your on-screen interface. The counter displays how many shots remain in the current clip.

The notches to the left of the counter display how many clips you have in reserve.

Ammo is universally compatible, but different weapons will use ammo at different rates.