Hmmmm i don't know where to start there is so much!!! Might be easier if I describe how I started my business up.
I started out just drawing for fun. I had a couple of friends and people on a local horse forum ask me to draw their horses, and from that I started charging very minimal costs (AU$30). At that point I was only 15, and was only working in graphite doing horses. I started getting busier through word of mouth as my skills increased, and I upped my prices a little. From that I got my first dog commission in graphite, and leading form that I got a few more dogs which opened up the idea of doing pet artistry as a whole. I learnt to use charcoal to a basic level, and charged a little less for a charcoal portrait than graphite.
After around 1 1/2 years of working ad lib through work of mouth, I created a basic website, with prices and examples of my work, then advertised on a few horse forums. This made my work load boom for quite some time and I upped my charges again.
When I was in year 12, I discovered chalk pastel. I won the top art piece in year 12 for a pastel of a horse, and I also won 2 out of 3 classes at the Royal Show for art, and came 2nd in the 3rd piece.
From that, I had people egging me on to up my prices considerably and go hard on advertising. I didn't advertise much at all last year so didn't get a heap of work, but towards the end of the year I ordered business cards, updated my website and put business cards and fliers around at local vets, pet stores, pet groomers, riding clubs etc. And my work boomed!!!
You certainly need to know the group which you are aiming for. Advertise according to your target market, don't start with high prices you won't get many clients. Start low, lower than what you think your work is worth so you are only really just covering costs with a small profit, but always charge postage!
Once you develop a solid client base, up your prices. Wait until you get a small waiting list, then up them again.
I go by the rule that if you have no clients, you're charging too much. A waiting list and you're charging not enough. A steady stream of work and you're just right.