1) Begin by looking at your old bicycle tire, and find the dimesions of the tire on its side. Write down these dimesions, becuase you must then go buy a new innertube for the tire that is the same (or about the same) size. I suggest going to a bicycle store; at the one near me tires run about 5-10 dollars.
2) With this new innertube in hand, you can begin changing your bike tire. You must start by taking off the entire bike wheel. To do this you need a hexagonal-shaped wrench thingy that looks like this:
See the hexagonal wrenches on the left side? You're ging to need something like that to remove the wheel. If you don't already have one, you can get this one from here
(Walmart.com), or you can look up 'bike tool' on Amazon or Ebay.
3) You then use this tool to remove two hexagonal bolts, one on either side of the wheel; shown here:
Then use a flathead screwdriver or another falt object/tool to pop the outer tire off to expose the old innertube inside, like this:
4) Remove this old innertub, and put the new one around the wheel. It might seem a little big, but that's okay. Proceed by slipping the outer tire back on over the new innertube, using the same tool you got it off with.
5) Put the wheel back on the bike, screw the bolts back in, and inflate the tire to whatever PSI (Pounds per Square Inch) it says on the side of the tire.
6) You're bike is ready to ride again!