The pet products biz. There are many pets and there are many pet products. You won't be able to underprice the big guys so you will need to develop a niche with unique or custom products. According to this American Pet products Association article, the following pet product trends are anticipated to be hot: earth-friendly pet products, branded pet products (for example a Harley Davidson carrier or Old Navy cat scratcher), pet friendly hotels and other lodgings, ultra-pet grooming products (electric toothbrushes for dogs), gourmet pet foods, programmable pet products (for example, programmable feeding systems, and self-cleaning litter boxes), fashion-influenced pet clothing, leashes, and carriers, and innovative methods of transporting pets (everything from portable carriers to motion sickness aids).
Do you have anything proprietary? Are your pet products decorated in a unique copyrightable way or have you invented something that might be patentable? Do you have a name or logo (for example, FURminator) that will grab attention? Can you create a YouTube pet video that will demonstrate your product's cool factor? Without a niche or unique products you'll have problems with your launch. Most importantly can you find a need that's unfulfilled and fill it? If not, you'll be slogging it out based on price with a lot of other hungry entrepreneurs.
The online biz. Setting up an online business is way beyond the scope of this blog. You'll need to learn about website development, SEO (or hire someone who does), or you'll need to join a community of online stores and hope for the best. You'll also need to move your website's Google page rankings from a 2/10 to at least a 5/10 to get the kind of traffic that will likely maintain a small online business. We'd quickly bore ourselves, our readers if we tried to restate all of the tiresome factors necessary to achieve success with an online store ... but there are tons of helpful books and websites that are just a Google search away.