Water
Find it, filter it, boil it.
You can go three weeks without food and three days without water. Find it first, then make it safe — always filter and boil if you're not sure.
The order
find moving water beats standing water filter cloth, then a real filter if you have one boil a rolling boil, one minute minimum carry two liters on you, always
The rule
Never drink untreated water because it looks clean. Giardia is invisible and unforgettable.