So, you want to climb Kilimanjaro, one of the Seven Summits and the tallest freestanding mountain in the world? Well, if you are going to be successful, there is a lot you need to do before you can even attempt to climb it.
You will need to go through a tour operator or expedition company. You can also go on your own and pick out a company while in Tanzania, but I am not to keen on recommending this just do to the uncertainty involved.
I went with KC Summits as the owner is a personal friend of mine and great all around guy. Just be sure whoever you pick is reputable, as your life is in their hands on the mountain.
If you haven't traveled very much in the past, or you have traveled a long time ago, you may need to get quite a few vaccines. Tanzania requires proof of a yellow fever vaccine upon entry into the country for example. While your tour company or expedition leader will give you all of the details regarding vaccines, just be aware that you will need them and that it can take time and money to obtain them. Don't save this until the last minute.
You will necessarily turn into a walking pharmacy for this trip. There is a lot of things you can potentially get: food poisoning, extreme diarrhea, vomiting, altitude sickness, etc. While your expedition leader will again fill you in on the most of these, be sure to consult a medical professional. It is much better to have all of the meds that need them and not have them. Make no mistake, this is not a walk in your local park. You will be in a developing nation at extreme altitudes while putting your body under a great deal of physical strain. One extra item I would take in addition to all of the recommended medicines is aspirin or baby aspirin to take on the flight home to prevent blood clots. This was something I was unaware of and be potentially fatal. Be sure to talk all this through with your expedition leader or medical professional.
You definitely need to be both physically and mentally prepared for the journey you are about to embark on. I had been training all year at the gym (get on an elliptical and set the resistance and incline to the highest available, it'll get ya) at Stanford, but I had about a week and a half before I left to get some real hiking in back in Phoenix on my favorite mountain, Camelback. This was great training for Kili. The trail gains 1300 feet over 1.2 miles and is quite a challenge. I did this 3 times in a row for training, and I felt perfectly prepared. That was perhaps the most interesting thing about Kilimanjaro, it doesn't demand that you are in the greatest physical shape of your life, but it will demand the best from your mental abilities. You are going to have to hike for 8 hours with a 15-20lb pack, getting up everyday at 6:30am, dealing with high altitude, and potentially getting sick. Physical fatigue may be the least of your problems. On the same token, being in good shape will never hurt you, and it will make your climb that much more enjoyable. It did for mine.
Here is where it gets a little bit more fun. Load up that REI membership, because you are probably going to need some new gear. While again, you will get a gear list, I will just briefly comment on the gear I brought along. You need to think in layers for this trip, as it will often be cold to very cold 30-40s in the morning when you wake up, and quickly warm up to around 65-75 during the day (during July).
Clothing - You are going to need a solid baselayer, Northface is a great choice, but I also found that underarmour was perfect for this as well. Top and bottom layers (thermal underwear). You are then going to need a mid layer, such as a fleece jacket or a lighter sweater. On top of that, you are going to need a solid jacket (down if you can) and insulated pants (I used an old pair of ski pants that worked perfectly).
You should also always have a waterproof shell either in your pack or on you in case it begins to rain, as weather can change very quickly on the mountain.
Another couple of items they likely will not tell you about in your gear list that I found useful:
A bandana - Some of the trails, especially on the way down, can become particularly dusty, and its helpful not to breath all of that in.
Handwarmers - Great for summit day, shove a couple of these in your gloves and even your boots if you can properly walk with them in, and you are golden.
Boots - In terms of the boots you are going to want to bring up there. While I bought a brand new pair of mountain boots, Asolo TPS520GV's from REI, you just need to make sure they are waterproof and offer enough support for your ankles. And the usual warning of wearing them in before hand applies here as well. Don't let Kili be the first time you are wearing these boots. The REI garage sale is a perfect time to find these kinds of boots, because a lot of people go on trips like this and return the boots after using them for a week. I think it is pretty pathetic, but what are you going to do? You might as well help out REI and save yourself a bundle in the process.
Poles - I may take a lot of flak for this recommendation, but I don't think you need poles for Kili. They even make your life harder in some places. However, they can be useful in places that are either muddy or wet. I would bring them anyway, in case you need them, but if you don't normally hike with poles, you will probably put them in your pack for most of the trip.
Pack - I used my trusty camelback for this trip to hold my camera, 3 liters of water, and a waterproof shell. This is all you should be carrying everyday. Not more than 15-20lbs. Your amazing porters will likely take care of the rest. I would recommend going against the grain on this one, and NOT buying a big pack, as this will basically ensure you carry too much stuff all day long, wearing yourself out unnecessarily.
Cameras - In terms of camera gear, I brought along quite a bit of stuff, simply because that is one of my favorite hobbies other than hiking. My main camera was still my trusty old Canon 30D, which is still taking great pictures I might add. I also brought along my new (relatively) Canon SD880IS for a backup and for occasion snapshots. On top of that, for a third backup, I brought along a Kodak waterproof P&S because I thought the cold might get to my other cameras on summit day. It took down my Canon SD880IS but my 30D managed to keep on trucking, so I didn't end up needing it.
Lights - I recently got a new Surefire E2DL flashlight, and brought that along. You need a solid flashlight and a solid headlamp. Just spend the money on something that will last you 20 years instead of the usual day for those cheap plastic flashlights. My headlamp failed over 15,000 ft on summit day and I used my Surefire as backup. By the best you can afford, because when push comes to shove, and you need it on summit day, you aren't going to want it to fail. I just bought a new heavy duty headlamp after this trip because of that.
Other than that, and the gearlist that your guide provides, that is pretty much it. Now on to the hiking!