Yes, about 5 guppies is all you should have in that 10g. It's a good rule of thumb to consider 1" of fish per gallon of water. Meaning for every 1" of fish in their adult form for every 1 gallon of water in your tank.
You should also consider a hood for your tank as well. This helps to keep dust and debris out of your water, reduces evaporation and keeps fish from jumping out of the tank and onto the floor. Hoods also have lights in them. A good flourescent tube lighted hood would provide the tank with light and protect the water at the same time. You didn't indicate whether you had a heater or not. If you don't have one, you'll need one of these as well. You can use a 60-75w heater for a 10g. That should be adequate. You'll also need a thermometer as well. I have two. I use a bulb thermometer that uses a suction cup to stick to the inside of the tank glass and an adhesive LCD strip that sticks on the outside. Water temps for Danios and Guppies should be in the 76 degree range.
It doesn't sound as if your tank has gone through the complete Nitrogen Cycle....This describes the process the water and tank must go through in order for you to successfully keep fish....
http://www.fishlore.com/NitrogenCycle.htm It's important to know that the fish can feel the effects of the cycle if the water is not properly treated during that time.
If you don't have one, you should get a water test kit. Most of us on here use the Aquarium Pharmaceuticals Freshwater Master Test Kits. They can be bought on line at several sources for anywhere from $15-$30 depending upon where you shop. I think that Petsmart.com is the least expensive at the moment.
When performing water changes with tap water, you must use a water conditioner. The recommended conditioner of choice on this site is called "Prime". You syphon the water out of your tank and vacuum a portion of the gravel at the same time into a bucket using an aquarium vacuum. In your case 2-3 gallons of water should suffice. You then add enough Prime for the amount of fresh water you are adding back to the tank into your "clean water" bucket. Fill your bucket with tap water as close to tank water temperature as possible...That's where having the bulb thermometer comes in handy....I use mine to regulate the temperature of my new tank water.
Slowly pour the fresh tank water back into your tank.....And you've just performed your first water change!! This is recommended once a week. But depending upon your bio load and water chemistry, more than once per week may be necessary.
For filter maintenance, I change my activated carbon cartridge every month. In between times, I swish it around in the old tank water that I just syphoned out. When putting a new cartridge in, I swish it around in old tank water before putting it into the filter basket. If using this type of filter and media, this works for me. I also check the impeller on the filter intake every other week and clean it as needed. You might want to try it if this is what you're using. Good Luck and don't be afraid to ask questions.
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Should I take the fish out first? Can I just put in tap water? How often should I change the water? I have a filtration system in place that seems to be doing well. Also, my one danio (I plan on getting two more very soon) seems extremely agressive (darting at my guppies) and whenever I even get near the tank he freaks out and darts about and jumps at the water surface to the point where I was forced to cover the tank. Is this normal? 
