First, consider what you intend to carry which is usually determined by the length of the hike, the location and anticipated conditions. An overnight stay with a restaurant or snack bar, for instance, does not require you to carry the same items as a weeklong trip through the wilderness.
You will need approximately one gallon of water per day, though it varies by activity, your size and the weather conditions. But, one gallon is a good starting point with which to estimate how much water you should bring and what size backpack you will need.
Outdoor vacations require planning just like any other vacation. Depending on what you are doing and duration, you may still need to find accommodations and make travel arrangements. Regardless of what details you must handle for your particular trip, every vacation requires proper preparation by determining what items you will need on your trip.