It's important to keep in mind where the bees you're about to purchase come from, as well as their race. Most beekeepers prefer to get local races, because this reduces the risk of bringing in new diseases or strains of viruses. If you're new to beekeeping, get bees that have a milder temperament, because this helps ensure you get stung less often.
Another thing that's important to consider is the timing of your purchase. Make sure you have everything ready before you receive the bees. For instance, have some food for them so they survive until they can start foraging. Make sure to have a look at a list of plants from which bees forage, and ensure you have enough of them around you all year round.
There are three main ways in which you can get bees: package bees, nucs and swarms. Package bees come with workers, food and a queen that's newly-mated. Nucs are made of workers, a queen and brood, in four or more frames. If you're a complete beginner, getting a package is better, because package bees have greater survival rates. If you're a little more experienced, nucs and swarms can also work. If you have the chance of getting a swarm but don't feel like you have enough experience, ask for help from a more seasoned beekeeper.