Best thing to do would be to get them copied down on CD (you can hire a studio for not too high a cost per hour if you dont have the appropriate hardware/software etc on your comp, and you can get session singers for a bit more if you or any of your friends cant sing - it doesnt matter too much but if the singing is decent theyre more likely to listen to it and of course - and importantly - it sounds professional!) MAKE SURE YOU HAVE IT COPYWRITTEN THOUGH!! then make some copies and send sample disks to advertising agencies explaining that you write jingles and wish to pursue this as a career, and would they be interested in contracting you or buying any of your work. Usually people who do jingles for radio etc are freelance - the ad agencies approach them with a concept which includes the product and how they want it selling (obviously you wouldnt want a happy tune for something grim etc!) and ask them to write a peice or song to go with it. And like any career in the industry its a case of you having to do a lot of work plugging your product and sending it to lots of radio stations, ad agencies etc before they begin to approach you since you have to build a rep and a name for yourself, but I think getting some of your stuff down on CD and firing it out to ad agencies (get the addresses off the net) is probably the best way to go. they might not want the tracks you send but it'll give them an idea of your capabilities, so send a decent range of stuff
I hope this is some help - and the very best of luck to you!