Just the same as most other services online.
SEO
Word of mouth is huge (not so easy when first starting out)
Work with local companies and friends first if possible then branch out to larger places.
1. Advertise on Forums
2. Making a Blog with Content on your web hosting company web site.
3. Make sure that your website and billing system look professional
4. Easy URL for your company so that people can remember it.
You can offer real cheap services to get some attention at first, bet in the long run you'll have to find another way to attract customers, e.g. specialty markets etc.
You can start promoting it to your family, then to your friends. After that you can create special package and coupon that you can promote in your local area.