Specific gravity is the measure we use to indicate how much fermentable sugars are in the wort or beer. Starting gravity, or sometime called O.G./original gravity, is the measure of sugars in the wort before it is fermented. Final gravity is the measured value after fermentation is complete. We get these measurements by using a hydrometer or a refractometer. If you do not have one, you should pick up a hydrometer. It is a "must have" for brewing and costs only ~$9.
Three main things will determine the flavor of the beer: malt, being in an extract or grain version, hops and yeast. You are looking to reduce calories in the beer and there are several ways to add flavor while reducing calories. Hops can be adjusted in quantity and varieties without added any calories. Too many hops at the beginning of the boil can make the beer almost too bitter, so some caution needs to be taken. Different yeast can also add different flavors. Yeast flavors can range from neutral to fruity and estery. Malt is the area that you will need to pay the most attention to since it will be the main/only contributor of calories to the beer. It sounds like you are doing extract with steeping grain brews. You should be able to dial back the amount of extract you are using therefore reducing you starting gravity(see above) to 1.035-1.040 range and with the use of steeping grains and the proper yeast can yield a very tasty beer.
Along with malt, hops and yeast, you can add spices to the beer to increase flavor. I have a belgium wit fermenting right now that I added a good handful of orange zest at flameout. The zest combined with the belgium yeast adds tons of flavor. The original gravity of this beer was 1.040 and finished at 1.012. This gives the beer 138 calories per 12 oz.
Along with the hydrometer, you should look into brewing software such as Beersmith. It will do all the math for you to figure how many calories will be inyour beer.
So to help you further, what kind of beers interest you? What kind of style would you like to shoot for?
