So you have a leak and have narrowed the problem to "there's a hole in the roof" (which might go something like this: "it leaks everytime it rains, there's a hole in the roof").
What you'll need:

-Tar (for rain and sunny weather ("wet+dry"))
-A roll of fiberglass netting (wet+dry preferably)
-Trowel, scraper, or stick that you'll never use again except solely for more tar involved jobs.


Step 1
Go directly above where the leak is on the roof and look around for a crack or a place where there might be a crack.

Step 2
Put a "glop" of tar over the crack, make sure it's enough to completely cover the whole and a piece of fiberglass netting (and then some). Spread it out evenly, but doesn't have to be perfect.

Step 3
Cut out enough netting to cover the crack and about 2-4 inches. Place the netting on top of the tar.

Step 4
Put another coat of tar on top of the netting so that you can't see any of it at all.

In about 2-3 years the tar should be dry. The fiberglass will keep the tar from cracking in the future as well (often overlooked by lots of people). To get any tar off of yourself just use some paint thinner or you could even use gasoline to rub it off.