If you have a tire with an inner tube, then regardless of the condition of the tire, something has also penetrated the inner tube and it is leaking air. First, carefully inspect the tire and see if you can find what penetrated it. Look closely for nails, screws, glass, even wooden shards, and whatever it is, pull it out.
If you can't find anything, you may have a malfunctioning valve core. Pull it out with your valve core remover:
A valve stem tool is a great thing to have in your tool kit, if for nothing else than expressing your distaste of car drivers who park in motorcycle spots.
Remove the valve core and let out the air. Clean it all up the best you can, and reinsert it. If you are lucky, the valve stem or core was a little off or dirty, and your tire can hold air.
However if you found the penetrating item, pull it out, and then you will have to patch the inner tube. Here is how to do that:
1. Remove the wheel. 2. Break the tire bead, and using tire tongues, flip up one side of the tire. 3. Pull out the tube. 4. Find the hole. Scuff up the surrounding area with the provided piece of sandpaper or file, and follow the recommendations of the manufacturer of your patch kit. 5. Once it is patched and dry, do a test to make sure its all patched. Spit on the patch and look for bubbles after inflating the inner tube. 6. Deflate the tube, insert it back into your tire, and re-seat the tire. 7. Inflate the tube and put it back on your bike.
Necessary tools: patch kit, pliers, air pump, valve stem tool, tools for removing your wheel, air pressure gauge.
Remember that this is a temporary repair. It should only take you as far as you need to go to get your inner tube replaced. Probably you won't have to replace your tire, even if it has a hole in it, unless it was badly damaged. I would not recommend the use of fix-a-flat for motorcycles. Some guys use it, but personally I would rather walk.