If you're slowly losing tire pressure, you may have a nail in your tire. Sometimes it may be difficult to locate the nail. Pour soapy water on the tire along the treads, and usually the leak will show itself.
Pull out the nail with a cutter or a pair of pliers.
Get a tire repair kit as shown. It's $10 from Pep Boys.
Use the the rasp tool to clean and roughen the hole. It may require some effort, so having some strength with your hands helps. :-)
Put one of the rubber strings on the needle tool, coat it with rubber cement, and insert it into the hole.
Pull the needle out, leaving the rubber string inside the hole.
Cut off the excess rubber string.
Pump the tire to specified pressure.