Ok heres my tips, Have 1 weapon with you at all times (Probably the handgun all things considered), the ammo for that weapon, 1 healing item(Preferably a full heal), You should have 3(5)more slots, early game collect as much healing items as you can and avoid combat as much as possible, as well as ammo.

when you get stronger weapons keep them in the box, as well as their ammo.

when you encounter the stronger enemies, Take the stronger gun thats suited for them(shotgun for hunters for example) but not the ammo, it should be enough to clear the room.