What can beat the feeling of those first steps onto the trail. Days of scenic hiking ahead and the self sufficient feeling of knowing everything you need is right on your back. Key to the whole trip is the right pack. With the information here, you can pick a winner.
Internal Frame vs External Frame
This is really in large part a personal decision. Internal frames have become extremely popular with many hikers, but external frames are still preferred by some, especially when carrying especially heavy loads. They offer a little better air circulation to the back and many models offer easier access to pack contents since openings are usuallly on the front rather than the top. It's also easier to attach gear to the outside of the pack.
Internal frames are carried closer to the body and give a greater feeling of balance. The frames of these can actually be bent to your specific contours for a true custom fit. For creek hopping, scrambling, narrow trails and fast hiking, they just keep it all together.
Getting the Right Fit
The first step is to know your torso length. You really can't guess on this based on your total body height. Tall people can have short torsos and vice versa. To get the right measurement, you just need a friend and a flexible tape measure like those used in sewing.
Have your friend measure your spine starting at the base of your neck. The point to begin is the largest bulge felt on the spine at the base of the neck. It's easiest to feel this when the head is tilted forward. Your friend should use the flexible tape to follow the contour of the back. The ending point is a line across the top of the crest of your pelvis. If you stand with your hands on your waist then move them downward, the crest of your pelvis is the bony arch you feel on the side. If you hold your hands at this level with your thumbs facing backward, the imaginary line between your thumbs will be the stopping point for measurement.
Armed with this info, you're ready to go pack shopping. Most quality pack manufacturers provide specific torso measurement ranges for each of their models. If you're faced with a small, medium, large selection instead, the decision isn't quite as clear. Different manufacturers can assign differnt lengths to these terms but in general expect lengths 18" or less to be a "small", 18" - 20" to be "medium" and 21" or above to be a "large". Choose a pack from a quality manufactuer, and one that is fully adjustable. You'll need to tune the fit once you have it loaded as we'll discuss below.
Even a pack of the correct length must be fitted to your specific frame. Most of these points apply equally to internal or external frames. Begin by loosening all the straps on the pack. You'll need the pack to be loaded for a correct fit. Preferably you need a weight similar to that you'll be carrying, and ideally the actual gear you'll be carrying. Tighten the hip belt first. It should actually ride around your hips, not on top of them. When hiking, you'll be carrying 80-90% of the weight on this belt, so get a belt that is adequately padded and firm enough that it doesn't fold up when tightened.
Shoulder belts should attach to the pack a couple of inches below the crest of the shoulders so that they actually follow the back down. In front, they should be of sufficient length that the buckles are well below your arm pit so as not to pinch against the arm or chest. Like the hip belt, they should be firm enough that they don't bunch up. Load balancing straps should come off the shoulder straps at the center of the shoulder or a little forward. Tighten these next. When hiking, these should not be over tightened. Use enough tension to just keep the pack in close. Stabilizing straps attach to the pack from the hip belts and should also not be over tightened. Lumbar pads, where applicable, should be adjusted to provide support to the lumbar area.
For internal frames, the aluminum slats within the pack (called "stays") can be bent to actually fit your contour. You'll need your friend again for this process to assess the fit of the pack while you stand in a natural position. For external frames, many are formed to a roughly anatomical position already. While the frame cannot be bent to fit your specific build, it should be adjusted up or down to most closely approximate your own form.
Once you're done, take the pack for a walk and give it a few shakes. See if it stays with you or tries to settle. Readjust as necessary. Taking this little bit of time up front will add immeasurably to your back packing enjoyment.