Anyone who whishes to own their businesses should be commended in my opinion. I whish I had done that when I had the chance, now, I have to start from step 1, again.
Rant aside
Having thought about a shop in the past, albeit photography and not diving, I was faced with the same issues that afflict most small shop owners: How do you compete with Internet sales and larger "brick and mortar" shops.
Pure sales is NOT going to get you there, even combining Internet sales with a shop may not get you there, at least for a while. ("There" is different for most people)
If you want a shop as a second source of income while you have a secure primary source, that's a great way to begin.
Considering the demographics of where you want to set up shop is also key, that will drive most aspects of what services you offer your local clients, these are the folks that will spread news of your shop around and recommend (or not) your services.
Consider overhead, things like rent or mortgage, utilities, local help, taxes, insurance, etc.
Consider sponsorship, local schools, activity groups, all dive agencies and their respective membership (PADI, NAUI, etc) and most importantly IMHO, travel agencies, unless you want to concentrate on local tech diving only.
Lastly, since sales alone will most likely not cover the larger part of your expenses, consider what services you will offer.
Classes, services and maintenance, fills, and general customer service. You may have to wisely take a small hit to your bottom line initially by going way above and beyond to ensure your shop is recognized as the place divers and those considering diving will think of first. Remember, you can replace a fin strap at the shop and you deal with the manufacturer directly; you can NOT replace that customer who left your shop permanently because you told him/her to deal with the manufacturer.
Lots to consider. Research wisely. Budget accordingly and do so conservatively.
Good luck !
Tanker