How do i contact advertising agencies?

You can create your samples and submit as portfolios to advertising agencies. If you need a lit of agencies in your area, you can check Yellow pages or search in the internet for local listings.

I'm not sure what type of advertising venue you have, but you may want to contact the end user like the marketing department of companies who would do the advertising. As far as an ad agency, depending on the size of it, you want to seek out the account executives. They are the one's who work with the clients and they will be the ones who most likely find the advertising mediums. So, call the ad/marketing agencies and ask to speak with an account executive.