Where do I start if I want to open an online business?

Even though your business is about selling pet stuff, I think you would get better answers if you browsed the different sections of YA and ask this in the appropriate place.

First you need to develop a website. Suggest you go to KimKomando.com where there are articles to read on getting started with one. Then you're going to need a list of products you want to sell, and where you will get them, and how you'll store them. You'll need some investment capital for your startup costs, advertising, technical consults, postage, shipping supplies, computer hardware, etc. For modeling, I suggest you review www.DrsFosterSmith.com, and www.thatpetstore.com, two of your bigger competitors..

You draw up a draft business plan complete with a SWOT analysis, then start looking for financing and suppliers, and take it from there.

You will have to buy supplies from wholesale distributors and then get yourself a website.