Because mycoplasma is so contagious, if even one animal in the group is sneezing or wheezing, you should reject them all. Once you've determined that the rats are healthy, the next thing to examine is personality. The best choice for a pet is an outgoing, friendly rat that isn't afraid to nibble your fingers or climb onto your hand. Although it's best to get a rat at 4 weeks old, when it is weaned from its mother, any friendly rat will make a good pet regardless of age. Many humane societies have homeless rats available for adoption, so consider checking there first.
Male rats get bigger and heavier than females do and tend to make better lap pets.
If you have a sex preference, be sure to examine the rats you are considering. To tell the sexes apart, compare several rats until you find two that look different. Because rats are so social, they enjoy living in pairs or groups. As long as each rat gets human attention, they will bond to you as well as to each other. A single rat will get lonely unless it receives lots of human attention, so in general, keeping at least two rats is best.