I would do exactly what raven said. If you find a large swollen area under the sore, gently squeeze it to see if any puss comes out. If it does, make sure you gently squeeze it at least once a day, pour peroxide in it, wipe clean and put triple antibiotic on it (neosporin) and rub gently until it heals. If you find it doesnt go away, or gets too hard to get under control, then you must take your cat to the vet or the infection can go to its brain and kill it. Keep up on it every every day and you should be ok.
You can basically treat it like you would a sore on you. Clean it and maybe use some peroxide and then neosporin. Vaseline will help keep bugs out of it, if that is a problem.