If you're starting out without design knowledge or experience, don't try and start a design company. It's highly competitive, and if you offer cheap prices, you'll get cheap clients (which believe me, you don't want!). As Chris said, clients should be coming to you because you have the knowledge and experience they require, not because you're the cheapest artworker around.
Start small, work for free for charities if you have to to get experience of working with clients and some work to build up your portfolio. Make a note of how long projects take you to complete, then work out an hourly rate, multiply these two together and that's your rough price.
Running a design business isn't just about being able to make things that look nice, you need to be able to deal with all the client shit that comes with it: Mistakes you make that cost your client money. Last minute changes that were needed an hour ago. Problems with the print finish. Clients wanting extra work and no extra cost. All this is stuff that you learn to handle from experience, but if you jump in with no prior experience, you'll find yourself out of your depth and disheartened.
I don't want to put you off (the opposite in fact), I just urge you to get some practical experience before you try to offer design as a professional service.
