Specific info on production techniques (like jingles at 10sec, 30sec, with voice over, with no voice over, just the hook, etc, etc.) would take way more time and space than we have here. There have been some previous discussions on this site, do a search.
I've got a couple of good books, but I' m not at home right now - I'll find them and post them tomorrow, but obviously scan the library and places like Barnes & Noble (or whatever large bookstores are near you).
Much of that type of work is done through "ad agencies" - well now they call them Marketing Agencies or Multi Media Agencies, etc.
So, one option would be to contact ad agencies in your area and see if they out source. The main problem is, now that recording gear is so inexpensive, most agencies keep that type of work in house.
Obviously, if you don't have a track record, you may need to create some dummy product (make believe customers) to show what you can do. Or even better, if you don't already have a portfolio (body of work). Create some spec. product - this means, create a message on hold for a local business (call dealership, insurance agency, etc.) and try to shop it to the marketing/sales manager. Also, contact some of the video production facilities in your area to see if they do corporate videos, etc.
I can tell you that I have done a fair amount of music on hold, music for private videos (weddings, etc), some corporate training videos, and even a couple of indie films - almost all of this work came from shameless self promotion and a willingness to accept rejection (much rejection).
But be forwarned, it is about much more than knowing how to compose music - you need to understand different audio and video media (SMPTE, etc.), you need to walk the fine line between artist and business person, you have to communicate musical concepts to non-musicians and you have to be willing to accept suggestions and critique from people who don't have a musical clue.
Oh, yeah and you have to be willing to carry a large accounts receivable balance, cause it's hard as hell to get some people to pay for your sweat and vision.
That all being said, it is worth the effort - good luck to ya.