Choosing Your Backpack…
In regards to your luggage, you must bring a backpack, and not a suitcase. This is the bag that will be checked at the airport.
Buying a backpack for your trip can be pretty overwhelming. You don't know the reality of how it's going to be used and, if you're like me, you have no idea what you're looking at when you walk into an outdoor store. If you know anyone with a pack, ask to borrow theirs. Packs can be pretty expensive.
I probably overspent. I got one of the best packs in the store. I also know several people who went for the cheapest option who wished they had spent a little more. My goal in this blog is not to make you an expert, but to give you enough awareness that you don't just freak out and buy the most expensive thing on the rack (like I did).
Internal Frame — This really isn't even an issue, but most sites I look at mention it first. Also someone tried to give me their external frame pack, so I thought I'd mention it. You want an internal frame pack (pretty much all you'll see in a store) rather than external (old school aluminum piping visible on the outside), like the one on the left.
Size — Anywhere from 60-70 liters should work. I personally think 70 is pretty big, but a lot of women on my squad had packs that big. The thing to remember is this: if you have the space, you're going to fill it, whether you need the junk or not. Smaller pack means less weight to carry. One of the biggest lessons learned from traveling the world is how little you actually need to live for a couple months. You'll make it work.
Fit — The fit of your pack is pretty important. The pack is one item I would by no means recommend buying without having at least seen the same model in person. Packs can be small, medium or large. The length of your torso makes a big difference. Different hip belts can vary a lot. Get with someone at the store who knows what they're doing. Outdoor stores are notoriously customer friendly.
Pockets and Zippers — There are three main places you're going to be able to access/load your stuff into your pack: [I'm not necessarily recommending this pack, but click here, for a thorough description of these three kinds of zippers/doors]
Most packs are going to have a "lid" on top so you can load stuff through the top of the pack. There will usually be a drawstring (like on the left) to secure stuff under the lid. The lid itself is usually a little pouch too. 
Many packs will have a compartment with a separate zipper for your sleeping bag on the bottom. There will be a removable "wall/floor" that will keep that compartment separate from the rest of your stuff. 
Some packs will have a zipper somewhere across/down the body of the pack so you can "front load". Front loading is nice so you can get to stuff without having to unload everything on top of it. Front loading won't really be all that necessary if you plan how you pack well.
