I have recently started working out, since August. I am 5'8 at 200 pounds. I don't know what is more important, aerobic exercise or weight-lifting. On a daily basis I can do one or the other but not both. At this point what is more important?

A combination of cardiovascular exercise along with lifting weights and sensible nutrition will be the most effective for weight loss and general fitness. Any one or two of these alone will not be as effective. If you are on a 3 day lifting program just do cardio training on the days you don't lift weights! You don't have to do both together in one day! Ideally, 3 days of lifting, 3 days of cardio, one day off. This sounds simple but it is an amazingly effective plan.