Depends what sort of beer belly you have. generally though, it all comes down to fat. You might not be "grossly overweight" but if you have a tubby middle, it's almost certainly down to excess fat, and getting your diet right is the most important thing.
Exercise alone will take a long time to shift it if at all - you only have to watch pictures of the London marathon on telly to see that even people who regularly run decent distances can still carry around significant extra weight. Unfortunately, though you may not like the answer, diet is the most important factor. reducing the amount of carbs you eat each day, particularly the refined sort like in bread and (sorry) cake, will really help. Try to get most of your carb intake from vegetables, particularly green ones, and avoid potatoes (in all their forms!).
beer is also a killer. a couple of pints on the weekend won't kill you but generally alcohol is high in calories and low in nutrients. If you feel like a drink during the week, wine, especially red wine, is a better option.
Sorry, but there is no magic wand solution and all the situps and crunches in the world won't give you a flat sixpack if your stomach is covered in a layer of fat.
in terms of exercise, avoid long duration cardio sessions. Shorter more intense sessions are better at boosting your metabolism and helping lose fat. Circuit training with weights is good if you have access to them. if you do go to a gym and use the bike or stepper, try warming up for a couple of minutes then do 1min flat out, followed by 2 mins gentle recovery, and repeat that 4 times. That 12mins of exercise plus warm up and cool down - will do much more for fat loss than 20 mins of steady pace cardio.