Deciding on the right model for your needs is perhaps the most important step in installing an air conditioner.
Calculate how large the room is where you will place the air conditioner. Be sure to include the adjoining room if the door will be left open and you hope to cool two areas. This square footage will help you to know how much BTU power is needed. That will, of course, drive what size air conditioner that you buy.
Other bells and whistles that you might want to add to your air conditioner are high energy efficiency ratings, digital temperature controls, variable fan speeds and settings for times when you sleep. All of these added features will increase your outlay.
Along with picking out the right air conditioner, think through where you want to place it in your home. An air conditioner placed in a room that receives a lot of shade can be a very economical move. If you insist on putting the unit in a room that gets a lot of sun, add BTUs needed to your total. You also need to add BTUs if you are placing your unit in a kitchen or in a room that has several people in it for long periods of time. Both of these types of rooms generate more heat than the average space.
Another consideration is where you will plug this unit in. You want to avoid loading up a given outlet or power strip that has several other appliances drawing on it. You also will need an outlet that allows for grounding.
Obviously, you need to measure your windows before picking out your air conditioner. You also need to think about escape possibilities in case there is a fire. No air conditioner is worth blocking your fire exits.