Congrats on your buying your first house, and what better baptism than a roof leak! (ok...bad pun!) :-)
There is really nothing you can do from underneath in the attic, except to try to locate the drip...but this is notoriously unreliable, as the water may be actually leaking under the roof up near the ridge, and then working its way down under the shingles to the point you are seeing.
Your best bet is to wait for the wether to clear, then go up on a DRY roof (holding a safety line that is securely fastened to something on the roof) and check the most obvious places where leaks occur. You don't mention what is on the roof, but fireplace flashing, dormer flashing and pipe flashing are the most common leaking points. The only thing you will know for sure it trying to locate the leak is it will be above where you see it in the attic. Flashing are the L-shaped metal pieces that go from the roof to the roof protrusion (chimney, dormer, skylite, etc) or for pipes, its the metal rectangle with the rubber piece in the middle that goes over the protruding pipe. Rather than try to explain here, suggest you go to your local library and look up roofing/flashing. Good books on the basics with good pictures and explanations, include Black and Decker, Better Homes and Readers Digest. At first glance, roofing seems massive and complicated, but it isn't. In fact, its one of the simplist structures in the house, relying almost completely on gravity to direct water to run off. If you keep this in mind, leaks are often quite easy to find. Once the leaking point is found, the fix is usually accomplished by reinstalling the flashing or repairing a ripped shingles with plastic roof cement from a caulking tube. If the flashing has come lose from the chimney, this will need to be renailed back in place and sealed with the plastic tar.
BTW, I hope you don't have particle board as a roof deck. Perhaps you mean wafer board (large wood flecks in random array) or Oriented Strand Board (OSB), with the wood flecks all pointing in the same direction. Particle board is made up of sawdust pressed together under high pressure using adhesives...but it will melt away when it gets wet :-( The little metal clip is used to join the two roof deck board edges when these edges don't meet over the top of a rafter....they are required by code, and should have nothing to do with the water leak.