Its not often you have to think about flooring, so you are not expected to know all the basic information, but a few tips can help you become educated and be able to walk into any flooring store and better explain what you want to the dealer.  Making the whole experience a little easier and less stressful for all parties involved.  
The first few things you should be considering when starting your search for carpeting are the use of the room, is it a high traffic area, how busy your families lifestyle is, and is there direct access to the outside?  If you have a good idea of these answer as well as if your family will be spending lots of time in this room, with children and pets running around or if this is an area with little use then the dealer can direct you in the right recommendations for your space and how it will be used.  

Once learning that basic information more direct questions will come into play, that a typical dealer will ask, are kids going to be playing in this room or will it be a formal living room?  A dealer would suggest staying away from light colors if this will be a room for kids, and they could also suggest choosing a carpet with BCF fibers like STAINMASTER, making clean up easier.  and less shedding of the carpet.

In small rooms choosing a darker color could make the room feel cozier, while going with a light color rugs will help make the room feel larger.

Lighting in a room can play a role as well, room that have lots of natural light will show the true color of a carpet, while if you have a lack of natural light will make a color appear darker than it truly is.  Lighting in most carpet stores will not be natural light, ask the dealer to bring home  sample and see it in your own home.

An extra tip- bring any samples or swatches of drapes, fabrics, furniture, wallpaper, paint sample or artwork that will be in the room.  It will help you see your range of color options that will match your existing decor.   Keep in mind that colors can affect your mood, warmer colors can make feel energized while cooler tones provide a sense of calm.

When getting to the cost of the carpet there are few things the dealer will ask, are you installing this carpet for you personal home and enjoyment or for the purpose of selling the home?  The dealer is trying to determine the style pr carpet.  Selling a home would warrant a neutral toned carpet in a simple style.  While if its for personal space you may want to look into the many varieties of style, texture, cut and color.

The dealer will then need to know the approximate size of the area you are planning to carpet so they can give you an estimate of cost for the project.   When you are ready to buy the dealer should send out a professional to measure you space for the final measurement.

The total price should include the carpet,  padding, installation, seaming, stairs (if applicable) thresholds, the moving of furniture, and the removal of old carpet or other flooring.  

Another tip is try and select the carpet from the highest grade carpet you can afford.  Carpets may look the same, but there are many grades to carpeting and your local dealer will be able to explain the differences in the carpets.   All carpets have different labels showing different mills and manufacturers, checking these labels will guarantee you are getting what you want and are paying for.