Choosing a pet rat is not as difficult as you may think. My experience is personal, and professional. Having grown up with both my parents being wildlife rehabilitators, and having spent the past 15 years with a no-kill animal rescue and shelter (www.merryangel.org), I have had vast experience with rats.
My first pet rats were with my brother, Seth. We had gone into a pet store, and seen the 'feeder' rats (feeder rats and mice-rabbits, too, are the ones bred en mass to be used as snake food). Being raised, as we were, we quickly rescued as many rats as we could pay for, and happily carted them home. Perhaps they knew they were being saved from a horrid fate, who knows, but the rats we kept for our personal pets, April, and Splinter (from Teenage Mutant Ninja Turtles, naturally!) were sweet, charming, and love from the start. (The others we placed in homes with our friends).
If you want to choose a pet rat, I strongly urge you not to choose a 'designer' rat, one that's bred for it's hairless quality, or other superficial reason, but instead, choose a plain rat from the feeder cage. Not only will you get a great pet, but you will also be saving a sweet creature from becoming a terrified food source!
When you come to the cage holding the rats, look at the rats and see when you put your hand in , how the rats react. If you are a novice to getting a pet rat, look for the ones that are not aggressive, but not timid, either. Rats generally are rather tame. You want to choose a young rat, as they will grow with you, and bond better. Also, you will have the pleasure of having your rat for a longer time. If you are a bleeding heart, as I am, look for the disabled, older, or downtrodden rats. Very often they express their gratitude with more than ample love and affection.
Another way to wind up with a pet rat is by chance. On occasion, I have found nests with baby rats. If you find a nest, it is best to leave it alone. However, that can be a mute point sometimes, if the nest has been damaged and no longer hospitable for the rats. In this case, while it is devilishly difficult, you can syringe feed 'pinkies', baby rats that are still hairless and pink. If the babies have their fur and their eyes are open, it is far easier to feed them, as they can eat and drink on their own at this point. Always keep your end goal in mind. If you plan to release the rats back into nature, keep your physical contact with them to a minimal... making them familiar and comfortable with humans will result in their death if they are released. If you are keeping them for pets, you are forging the bond with the rat(s) at an early age, and this will result in a deep, and abiding loving relationship throughout the life of your rat(s). Always remember, a wild rat is wild, and they can be aggressive (self-defense is to be expected) and they can, as any animal can, carry diseases.
Choosing a pet rat is not complicated. Look for the things you want, the colors, gender, and attitude. You want to choose the rat that will fit with you, and be a part of your life... not just a furry rodent you look at and feed occasionally.
Rats are really intelligent, and very social. Once you have chosen your rat, give a few weeks for the two of you to get to know each other... you're going to be surprised at just how involved, active, and affectionate your new friend will be!