I've banked several bike shops in my time. Some have been 'proper' businesses and others little more than a hobby Invariably the ones that do well have territorial exclusivity of top brands. Specialized, Marin, Cannondale etc and cater for ALL types of cyclist, kids, roadies and families. Find out first which brands are available in your area. IIRC some brands such as Marin are much more fussy about how many shops they supply than others such as Giant.
Stock control is key. Stock will take up most of your cash and you need to be able to turn it around quickly – so as much as you would like to stock top end full bouncers be careful. These are expensive lumps of metal that won't do you any good as a showpiece for kids to gawp at after school. Premises are important – you'll need parking etc and research the competition.
Forget about providing niche stuff – the internet has won that battle