NO NO NO. do not start a business espec. brick n mortar where you must cover a lot of fixed costs (as previously mentioned) - rent, insurance, inventory. as some have suggested start out of your garage or basement or even your spare room in the house (if you have those luxuries of space).
selling online is tough cuz you have no established reputation.
also, has some have suggested, you need to get more small biz info whether online, local advisors, local bank, etc. as also stated opening a business is to make money not make hobby of it.
do some market research (can your local community sustain your business, is there a demand - not just one or two who think it's a good idea & they'd be your 1st customers type of deal), how many households have aquariums, what is the potential for new starters, etc.
come up with a business plan (which includes not only cost estimates, fixed & variable - i.e. rent includes security deposit, construction, fish/non-fish supplies, inventory, utilities) but also # of & size of tanks, types of common easy to sell fish, what supplies/brands to carry, water supply, work/prep area, etc.).
economy is still in a recession - so fish keeping is a luxury that many may/can start but may not grow with you to sustain your business.
as one stated, hidden or unexpected costs (fish loss, not just one or two, or disease breakout) hit you & your hobby is a lost cause no longer fun.
if you can breed some fish & sell online/locally that be a good start. consider some of the easier fish - spark some interest.
but if you do decide I would anwer your inquiry as follows:
1. select space where rent is low (best in your own home somewhere) & can allow for expansion, security deposit, utility costs, construction costs, advertising, phone, taxes, vendor/suppliers, storage space, etc. etc. etc. by no means an all exhaustive list needed.
2. # of tanks --> size of tanks --> types of fish --> quantity
3. filtration - try multi central system each for fresh water community tanks, cichlids, discus, feeders, goldfish/specialty golds related. this way they won't cross contaminate each other when disease hits. or a combo set up, some on central, some on their own separate filtration.
4. QT tanks (unlike the large LFS & many local LFS - they just float the bags then dump)
5. make list of all supplies to carry from tanks, filters, heaters, lighting, meds/water treatments/test kits, foods, gravel/decorations, plants, accessories, maintenance supplies, plastic bags, etc. etc. etc.
6. get all your tanks cycled.
7. then order your fish, supplies
8. then observe their conditions to be sure no disease outbreak.
9. treat-QT asap
10. after a week or 2 open your doors & wait for them to come rushing in.
I don't think I covered all the steps but hopefully you get the gist. We all wouldn't want you to bite off more than you can chew.
but good luck either way.