You may not need to lower your pH, depending on the fish you'd like to stock. If you're wanting tetras and angels, then you probably will need to make some chnages, but you'll need to do so slowly, and the best method will depend on your water conditions (GH in particular).
Some basic method:
Using peat filtration in the water, or adding driftwood. Both of these add tannins, which will lower the pH--the exact amount varies with the amount of peat/wood used. This method will result in a tea-colored stain to the water that is harmless, but may be unnattractive--it's easily removed with carbon.
Mixing tap water with RO water will also lower the pH, since the RO water will be pH 7.0, so a 50-50 mix should give you a pH around 7.5. This isn't as convenient, and obtaining RO water (unless you have a RO filter) can be a pain. You'll also need to experiment to get the correct ratio, and make sure that you retain enough buffer in the water to prevent pH crashes.
Check
http://www.skepticalaquarist.com/ for both more information on changing pH, as well as methods. Keep in mind that unless you're looking to breed the fish, most fish can be acclimated to a variety of pH, and all fish will do better with a stable pH than one that jumps around.