If you are still early in your undergrad career, consult the economics advisor at your school about what courses to take, with a plan to be prepared to take at least one or two graduate courses by your senior year. That will help you understand whether you really want to go to grad school in econ, and if you do, success in a graduate level course or two will help demonstrate your capacity for successful graduate study. A dual major or a minor in some cognate field (math, CS, OR, stat,...) is also a strong signal, and good preparation. (And econ could be your minor if one of those is your major.)
Assuming that you have compiled a strong undergraduate record, the (next) most important part of your application will be your letters of reference. The job of your letters is to convince admissions committees that you can succeed in their Ph.D. program. So the best letter writers will be those who know you well, and who also know what it takes to succeed in a good Ph.D. program, so that they can write persuasively that _you_ can succeed. A letter from an assistant professor who is himself a Ph.D. economist and has been your teacher in a demanding course in which you did well is often far more effective than a letter from your boss at your summer job, or even your supervisor at your current job if you've graduated. It isn't so helpful to hear that you are smart and hardworking if the letter writer doesn't know what talents you need to succeed in your first year Ph.D. courses and beyond.
Admission to Ph.D. programs in Economics, particularly the best known ones, is very competitive. So, almost no matter how successful your undergraduate career has been, plan to apply to multiple programs, since there's a biggish element of chance in getting admitted. And there are lots of universities at which you can get a good education in economics. So if your undergraduate record isn't stellar, or you are at a school that hasn't sent a lot of graduates on to econ Ph.D. programs in the past, but you have an idea of what kind of economist you might like to become, you can find good programs that may be prepared to take a risk and invite you in, especially if you can explain in your personal statement why they should. ( The very most competitive programs don't generally try too, or have to, take a lot of risks...).
Do spend some time figuring out if you really want to be an economist, and whether you are prepared to devote five or six years to getting a Ph.D. There are lots of ways to have a fulfilling and valuable career that don't require a Ph.D. in econ. (Most people don't have one:) Getting a Ph.D. involves jumping through some hoops, not all of which will be equally enjoyable or even necessary for the kind of economist you want to be, so a big part of your preparation for applying should be figuring out if this is something you really want to do. If it is, it can be the entryway to a very rewarding career. (And given what we currently understand about economics, we could certainly use your help:)
