However, you're going to have to clean up the gravel more.  While scooping the water out will get the water, it's not touching all the gunk that settles into the gravel.

As soon as you can, go back to Petsmart or whatever petstore is nearby.  Ask someone who works there to help you if you need to.  Look for something called a gravel vac, or a siphon tube.  You can see a picture here... http://www.petsmart.com/global/product_detail.jsp?PRODUCT%3C%3Eprd_id=845524441776778&FOLDER%3C% 3Efolder_id=2534374302030123&ASSORTMENT%3C%3East_id=2534374302023693&bmUID=1147125752103& ;itemNo=5&In=Fish&N=2030123&Ne=2

You can get something as simple as that, or some really complex things too.  There should be instructions on the package, but in case there aren't, here are some for the simple siphon tube.  Before you do this, make sure your hands are clean, and if you use soap, make sure all traces of soap are gone from your hands.  Rinse really well.  Soap is like poison to fish.

1. Remove any bigger decorations from your tank and set them aside in a dish or something similar.  You don't have to do this every time, but if it's the first time you've cleaned your tank, you'll probably want to.

2. Have an empty bucket ready, at a lower level than the tank is.  (Read: Probably on the floor.)  It doesn't have to be perfectly clean, but it'll make things easier and more sanitary if it is.  You might also want to have towels around on the floor, just in case.  It shouldn't be messy, but you never know.

3. Submerge the entire hose in your aquarium water.  Make sure there aren't any big bubbles.  You may have to try it a few times.

4. Put your thumb over the narrow hose end and pull it out of the water, making sure not to let any air into the hose while you're doing it.  Point the end into the bucket.

5. With your other hand, get a good grip on the "vacuum" end.  Keep it below the water and try not to suck up the fish.  It won't hurt them, but I'm sure it's stressful.  They'll probably stay away anyway.

6. Let go of the opening with your thumb, and water should flow into the bucket.  Poke along the surface of the gravel, aiming for any dirty spots you see.  Don't go digging all the way to the bottom of the gravelbed, because you'll disturb the good bacteria.  You're mainly going for fish poop or uneaten food.  This decomposes and will raise your ammonia level.

7. Stop if the water gets to less than 50%.  Even if the gravel's still dirty, you don't want to change any more water than that at one go.  You can always do it again in a week or so, or sooner if you really have a problem.

8. Make sure you refill the tank with room-temperature, conditioned water, preferably some that has set for a day, though I personally have never done that and my fish don't seem to mind.

9. The old fish water is actually pretty good plant fertiliser... toss it on some flowers or something.  No need to waste it. :)