Sounds like you may need new tubes. Tubes may be purchased from bike shops, most hardware stores, and some big box retail stores like Kmart and Target, to name a few.
If the leaks in the tubes are reasonably small, there are several products that can be squeezed into the tubes to seal punctures - both existing ones and ones that occur after the product is added. They are put into the tube through the valve stem, similar to adding air). Two that I know of are Fix-a-Flat and Slime. Both can be purchased at most hardware and automotive stores.
Changing a tube is not terribly difficult but can appear daunting if you've never done it before and don't have some of the tools required.
Hereare step-by-step instructions for most styles bicycle:
- Loosen thenuts that hold the axle to the frame.If they are extremely tight, spray some lubricant on the nuts - a siliconelubricant or even vegetable oil will do the trick.
- Take thewheel out of the frame. If it's therear wheel, you will need to lift the chain clear of the gear cluster. To easethe removal of a rear wheel, shift the chain to the smallest gear on the wheelbefore loosening the skewer or nuts. If it's the front wheel, that one will bea little easier. You may also need to release the brakes if they interfere withwheel removal.
- Deflate thetube completely by pressing down on the inner part of the valve.
- Take acouple of tire levers (you can purchase these at your local bike/outdoor store). You can use the handle of a spoon or similarobject if you don't have tire levers but be very careful, as you riskscratching or damaging the rims of the wheel and/or puncturing the inner tube.Ease one lever in under the wheel rim and lever out the edge of the tire(taking great care not to puncture the inner tube) and pry it up over the wheelrim. Move around the rim about an eighth of the circumference and repeat theprocess again, leaving the first tool in place. Now zip the second lever aroundthe wheel and the tire should come right off on one side.
- Remove thewheel and tube completely.
- Replace theinner tube
- Check thetire wall for an arrow or similar to indicate the direction of rotation - sometires have a "direction specific" tread pattern.
- Put one sidein first, then ease the partially inflated tube into the tire and locate thevalve in the hole in the rim.
- Starting atthe tire edge closest to the valve, use your thumbs to work the other side oftire over the rim and into well.
- Beforeinflating, use your thumbs again to ease the tire from the rim all around the circumference,peeking in to make sure that the tire is not pinching any part of the tubeagainst the rim. When youinflate the tube, if it is pinching, it will pop, and you will have to repeatthe entire process, and buy a new tube.
- Inflate thetube,checking to make sure the tireis on evenly and there is no "pinching".
- You're now ready to put the wheel back on the bike.
Hope this helps.