First things first, always consider the purpose of using your tent before buying it.
For example, if you expect sunny weather on a camping trip, you might consider using a sturdy tarpaulin as your shelter.
But as we all know, the weather is quite unpredictable so also think about buying a three season tent.
These varieties can withstand mild winter conditions (if you plan to camp in winter), more often in the summer. If you intend camping in extreme places, such as an expedition to Mt. Everest for example, four season tents are the way to go.
Weight must also be a major consideration when buying a tent. Camping always involves hiking, and that can be a drag around a heavy army tent that can load ten on your weekend camping trip with only 3 of your best shoots. Try and find the lightest tent that suits your purpose. Tent bets, or pegs, also add a fair amount of weight to your package. A military tents with fewer bets is better in terms of weight, but those who have more bets can withstand much worse weather. So always think of the weather.
Check the tent fly before buying it and make sure it is completely covered. A fully closed zipper will make you and your belongings dry in case of rain. Some army tents also come with a separate waterproof section where you can store your bags and boots. You might want to consider buying this type of tent if you don't want to be in a tent with your belongings. Plus, it always pays to keep your boots and clothes dry, just try walking for miles with wet socks. This is no fun.