Advertising is a tough field to break into, but don't give up. I recommend that you get some kind of job outside of advertising while you're looking. I believe that a paycheck will be more rewarding financially and emotionally than just looking for a job. I believe that having a job will also create a more favorable impression with prospective employers in the advertising field.
People employed in the advertising have one or more of the following skills: copywriting, graphic design, sales, media, web design, database management, direct mail and trade shows. Another field closely related to advertising is public relations. Since you haven't had the opportunity to develop those skills, you need to find an entry level position--perhaps an administrative assistant in an advertising firm.
Keep in mind that there are different forms of advertising, i.e., consumer versus industrial. Ad agencies provide a variety of services including ad placements, copywriting, graphic design and media buying. There are also freelancers that provide these services. In addition, there are large companies that have in-house advertising departments that perform some are all of the same services offered by ad agencies and other service providers.
I am a firm believer in using the internet. Try Googling "careers in advertising" and "jobs in advertising" for starters. You can also try "advertising trade associations." You can contact these trade associations and ask them if they can provide any resources for individuals seeking a career in advertising. if you can obtain any lists of advertising firms with key contact info, you can use a shotgun approach by mailing your resume with a nice cover letter.
I hope that the above information will be helpful in your job hunting efforts.
Feel free to let me know if you have any questions.