Start with finding a source of fresh beans and a local one if possible. I always try to encourage people to find a local roaster that they can walk into and talk to the owner(s), barista(s), and roaster(s). They will be able to tell you about the beans, how old they are, the roast level, the flavor profiles, and they brew it, etc. There are good online sources, but I think local is better and usually fresher. 

Single source beans will give you more defined flavor than blends. Blends can be good, but if they aren't blended well, you'll get uneven extraction that could alter the flavor adversely. 

Don't get your beans from a grocery store and most of the major brands aren't very good. Beans from these sources are usually at least a month old, and for some of the brands even a year old before they even hit the store shelves. Beans start getting stale immediately after roasting and by a month they will lose most of the good flavor and pick up a lot of bad flavor. 

For the best espresso results, try to get beans within a couple days of roasting and try to use them before they are about ten days old. 

- Joe