Before you even visit the pet or aquarium store, first make sure you can answer yes to all of the following questions:
Do I have a little bit of time every week to clean my bowl?
Do I have a small space for a bowl that won't be too disturbed, especially by other pets and young children?
Have I done my research?
Am I a generally responsible person who will do a good job caring for a living thing?
Often people will buy a betta and forget about it. If you have any hesitation about whether or not to buy a betta, do some more research to see if this hobby will work for you. If you know that your betta won't be well-cared for, don't hurt your betta by providing it with an insufficient home.
Before you buy your betta, set aside some kind of small storage container to keep all of his supplies in one convenient space. I use a plastic shoe-box-sized storage container to house extra food, water conditioner, extra pebbles, nets, cups, a rag, and more which fits easily under my bed. (I usually keep the food out since I use it every day.) That one box holds all of my supplies for four bettas.
Also designate where you will keep your betta's home. Do you have room for a three gallon tank or something larger like that, or do you have small space for a gallon bowl? Think about what kind of light comes into the room; your betta should have plenty of natural light but shouldn't be in direct sunlight.
I also strongly advise testing your tap water before buying a betta. Most people find theirs just fine, but sometimes you'll find that your tap water has a low or high pH, something that needs to be regulated to neutral (7.0) for bettas. You'll find additives in the pet store that will neutralize things like pH and hardness. If you find that you need these, buy them on your first trip.
When you find that you're ready to buy a betta, choose a day that you will have a few hours to go to the pet store and then establish your betta once you're home. Bettas are the best established when their owners have taken their time in introducing them to their new home. Keep in mind that if you're keeping your betta in a filtered tank, you may choose to cycle the tank first. There's a great page about cycling here. Cycling means that you'll only have to do 25% water changes each week instead of 100% water changes (what you'd do if you don't cycle). Cycling refers to establishing the nitrogen cycle in your tank wherein you cultivate beneficial bacteria that eat most of the fish waste and so are essentially keeping your tank clean for you.
Anyway...
Now it's time to go to the pet store! Your store should be well-maintained and have employees that know about the animals they are working with. Unclean stores often buy cheap bettas from unreputable breeders that may not live long. Along those lines, I've found that Walmart is not a good place to buy bettas; they are often sickly. Once you have found a store, find the aquarium seciton, which often has an entire section devoted to bettas. They might keep male and female bettas in separate places. Males have long fins and are often bigger than short-finned, small females (read more about the differences between males and females here).
The bettas will probably be in small, clear plastic cups. Some may have blue liquid inside. This is an additive that petstores put in cups to keep their fish from contracting diseases (it's called Methylene Blue; learn more about it in the First Aid section). Also keep in mind that if the betta takes particular interest in you, he or she may be worth buying. Sometimes the betta chooses the owner, not the other way around! Look at the following guidelines to find a healthy betta.
betta has color and is not gray
eyes look clear and move often
fins are intact; no tearing or ripped fins
responds slightly to your movement
seems alert and interested
no scrapes, cuts, or tumors on body
[When you get to be an experienced betta keeper you may decide to experience the joys of buying a sickly betta and nursing it back to health! Don't try to do this with your first betta though.]
Bettas exert a lot of energy flaring at each other when they can see each other. Often, the bettas in the stores will seem sluggish and tired simply because they have spent so much energy making faces at each other. Most will gain energy once they are isolated and established.
What else you should buy on your first trip:
small brine fish net: this soft net is not made of plastic, so it won't cut the fish. It should be rather small. Many are labeled "desktop net" in pet stores.
betta pellets (I recommend Top Fin brand. See Food for more information)
water conditioner, preferably made especially for bettas (see Your Betta's Home)
tank pebbles/glass stones (rounded stones only: no sharp points or gravel! see Your Betta's Home)
tank ornament (see Your Betta's Home)
some form of test kit / additives if your water quality needs changing
if you don't have one already: a betta bowl or tank (see Your Betta's Home)
After you purchase these items, it is crucial that you get home immediately. Bettas can easily perish if they are subjected to a long ride home. Temperature fluctuates the entire trip home, and bettas who are not established die easily from shock. If you need to run errands during this trip, do so before going to the pet store.
Now you are ready to move to Establishing Your Betta.
Further Reading:
http://ezinearticles.com/?What-to-Look-For-to-Buy-a-Healthy-Betta-Fish&id=285829 <---fabulous resource!
http://www.bellaonline.com/articles/art35928.asp
http://www.aquaticcommunity.com/bettafish/buying.php
http://www.bettatalk.com/buy_bettas.htm
http://www.aquariumfish.net/catalog_pages/bettas/betta_males.htm
http://www.essortment.com/all/bettafishbuyin_rjen.htm
Further Resources:
Example bowls, homes, and kits:
http://www.petsmart.com/product/index.jsp?productId=2753229 <---I use this with one of my males, but without the included stones and plant--they are too rough.
http://www.petsmart.com/product/index.jsp?productId=3806240 <---I use this with my small female, but without the huge plant. :)