It is very difficult to get involved with "jingles", the major portion of this work runs through marketing firms - and since quality recording gear is so affordable, most agencies now have "in house" studios (I've lost much of the work I used to get from agencies). In addition, most radio stations have studios to produce jingles.
However, if you have the "chops" and the willingness to "do whatever it takes" there are ways to break in.
1. Create "spec" jingles. By this I mean, identify companies in your town that alreading run radio ads (not McDonalds, but things like car dearlerships, plumming supply stores, nightclubs, etc.) create a radio ad and bring it to them. Ask to talk to the sales/marketing manager. They may like it or they may refer you to the agency they use.
2. Approach the marketing firms in your area and find out if they record in house - or contract out. Be prepared to present samples of your work.
Be prepared for lots of rejection (just like a song writer). And be brutally honest with yourself regarding your jingle chops. There are many talented people fighting for this type of work.
Also, learn the formats and recording medium the local radio stations need. Keep in mind most jingles require several "verisons" (with script, without script, full script, just the hook, 15 seconds, 30 seconds, etc. etc.)
Lastly, vocal talent is the single most important requirement - If you need to bring in vocal talent (singers, people who can do accents, etc) it can eat up much of the budget.
The more self contained you can be, the better chance of actually turning a profit.