The next question is the size of the backpack, which will depend on the type of trip, the conditions, and the equipment needed. Get a backpack with enough space but remember, if you take a backpack that is too big you will be tempted to fill it up! And a bulky backpack restricts freedom of movement and is more difficult to carry. Look for a pack made with durable, ultralight materials and a simple design. These usually have a large roll-top main compartment, minimalist padding, and exterior pockets for water bottle and things you need to hand during the day (hip belt pockets are handy).
Do your research and read the product descriptions carefully to make sure you are getting the right backpack for your trip.