Most jingle writers work for jingle studios or ad agencies and have degrees in advertising or theory and composition. Of course, if you've got a Pro-tools studio and know how to sequence, record, master, and hire out anything else you need, you can work without a degree and even bypass the ad agency.
The easiest way to break into the jingle business is on the local level. You might have to give a few away to establish yourself, but once you get some stuff on the air, you can make decent cash while working your way up to the regional and national levels.
The practice of licensing established popular songs for jingle use has become common. Hit songs can sometimes generate more money as jingles than they did from airplay. Some bands have even gone the reverse route, having a song that was licensed for jingle use become a hit due to the exposure from radio and TV.