Hello and welcome to the fourm. First off I would advise you to read the beginner's resource center (Drobbyb has linked it in his post), it is fishkeeping 101 great for beginners - I lived in that section for quite a while. Espically read the section of cycling a tank, it sounds as though you are in a Fish-In cycle, which means a lot of water changes and tank managment. If you can return your fish that would be the best and complete a fishless cycle - a lot less work. Also invest in a liquid test kit. I use API master kit around $30 but well worth it. The strips are just not accurate, trust me I made that mistake when I first started, it is not worth the hassel just get a liquid test kit. As for your questions:
1) How much below the top of the tank should the water level be?
This is some what personal preference. For my Betta tank I fill the tank almost to the top because I find it slows the current of the filter which is better for the Betta. For my other tank I fill
it a litter lower, the current from the filter is a little stronger with the water a little lower, my danios just love to play in the current
2) How much food should I be giving the fish?
I was told feed only as much as they will eat in a 2 minute span then remove any uneaten food. Do this twice a day ... this seems to work great for me.
3) How do I test the ammonia level and how often should I test?
Best way is with a liquid test kit. Once your tank is fully cycled and established I think once or twice a week would be fine. But for right now with you being in a fish-in cycle you want to test
at MINIMUM once a day ... but really should be roughly every 12 hours. You also want to test for nitrite, nitrate and pH using the same schedule. All these tests are available in the API
master test kit.
4) Is the Whisper EX30 filter sufficient?
Sufficient ... yes for your tank size the EX30 will filter upto a 30 gallon tank. Filters are really personal preference. For me I don't like those filters because there is only one filter cartridge and you really don't have a lot of space to adjust (change/add) your filter media for your tanks requirements. Also I don't find that they have great biological filter media. Everyone I know who has had a tetra whisper filter, including my self has returned it. You may want to look into an Aquaclear 30 (the model you would need for your tank). The media goes into a basket so you have total control of what type of filter media you use. Also you can make your own filter media bags for very cheap, but the replacement filter media bags and sponges made by AquaClear are very inexpensive - around $5 for 3 replacement carbon bags - so essentially filter maintance is very inexpensive and you can adjust you media to your tank requirements. Also it has a separate biological media bag - again you can use what comes with the filter or you can use a different type of biological media depending on your preference. The initial cost of the filter is a little more but the inexpensive maintaince and freedom to control your media more than makes up for it. It also has adjustable output flow, so you can decrease it if it is too strong for your fish. The AquaClear 30 (for a 20 to 30 US gallon tank) is approximately $30 US. Again this is just my personal opinon I have the AquaClear 20 for my 5 gallon tank (just upgraded a few days ago) and I love it. But read up on filters on-line and find one that you and your fish like. In the meantime the Whisper EX30 is fine for your tank, just as you get more experienced you may want more freedom with your filter media.
5) What is the best way to tie plants to the driftwood?
This I don't have an answer for as I don't use driftwood, but the previous suggestion of dental floss or fishing line seems good. Just if you use dental floss make sure it is unflavored and undyed and they don't use chlorine to bleach the material as this will certainly cause problems in your tank. If it was me I would just go with fishing line.
6) To add water to the tank, should I let the tap water sit for 2 day. with water conditioner?
You should use a dechlorinator/conditioner with your water. If you use it you don't have to let it sit for 2 days, but read the instructions to make sure you dose correctly. For me I always have 6 gallons of treated (dechlorinated) water on hand for emergency water changes. So I do let mine sit in the water dispensers. Walmart sells 3 gallon water dispenseres that work great they even have a spout for easy dispensing. But if you are using a dechlorinator/conditioner you don't have to let it sit, just make sure you treat the water before adding to your tank, and only treat the amount of water you are adding to your tank, ie. if you are adding 1 gallon of water treat the 1 gallon then add.
Hope this helps and good luck.