A buddy of mine started by doing a mobile detailing company - he would go to your car, parked wherever. He got busy enough that he ended up renting an unused garage bay at a used car dealership, and they gave him a cut on rent if he did a quick vaccum and wash when cars came in, and on delivery...
A year later, he was busy enough (from referrals, and word getting out about the quality of work) that he now has taken out a lease on a 3 car bay building (that used to be a used car dealership). He's got two dealerships in town that use him for detailing work, plus all the other work. Can't keep up.
As mentioned, start slow - do SOLID QUALITY work, and keep it affordable for people in your area.
After a year or two, re-assess the business and go from there.
Another good idea is to go to other detailing company sites and see what they charge. If you're new and looking to build - you can't expect to get what other may be asking at this point in time... but it should give you a "ballpark".