Ich usually starts in the gills, where you can't see it. The fish may seem a bit frantic and rub itself on rocks and decorations since its gills are itchy. When it's full-blown ich, it will look like little grains of salt on the body of the fish, where the parasites are hatching out of the skin. Larger white spots may be another type of infection.
Unfortunately, no matter what cure you use, you may still lose fish. You can try to up the temperature in your tank, or use a chemical treatment like malachite green. You'll also want to vacuum your gravel and do 20% water changes several times a week to remove the eggs from the gravel. If using a chemical treatment, make sure to remove any carbon from your filter, as this will prevent the medication from getting to your fish. Whatever chemical treatment you choose, be sure to follow the package instructions.
I tried to quarantine the first fish that I saw lesions on, but then it became apparent that everyone had it. You should probably try pulling the sick guppy in case it really is the only one infected, but be careful not to stress it out further (like making sure your quarantine tank has the same temperature and water conditions).
For what it's worth, an experienced fish keeping friend said the increased temperature can actually be harder on a sick fish than chemical treatments. Plus the increased temperature may make the fish more susceptible to a secondary bacterial infection of the little white sores.
I hope this helps, and that someone more experienced than me (that is, not very experienced, but having done a ton of research when my tank got ich a few weeks ago) can elaborate or correct any incorrect info.
Good luck!