What sort of recipe did you use? The factors that controll alcohol level are the amound of fermentables you have in your wort at the start of fermentation, and the degree to which they are fully consumed by the yeast. So either there wasn't enough malts or other sugars present at the beginning, or the yeast didn't burn through it all. Was the beer very sweet tasting? It can indicate an incomplete fermentation. Also, what sort of yeast did you use? certain yeasts ferment dryer (less sugar, more alcohol) than others.
If the beer is low in alchohol content, test your wort to check the original gravity before taking off the boil. Be sure to cool the wort you will measure down to about 70 before testing, or you'll pop a hydrometer. Depending on that reading, you can adjust gravity by adding a bit of dry malt if you wish. Having enough sugars from the mashing process coupled with a good yeast should improve alcohol outcomes. If doing AG, the efficiency of your mash will help determine alcohol levels, as well. Finally, give the beer enough time to fully ferment. Beer not fully fermented will produce a sweet taste that can be over-powering.