To set up the website and get it off the ground, unless you have the expertise yourself, you will have to pay for this service from a professional web design company. These companies will know how to incorporate services that allow customers to buy and pay for goods on the website.
Companies will basically sort the site out for you yourself and you don't have to worry about it.
In terms of legally setting up a company-
If you work on your own and are self employed then the likelihood is that you will need to register yourself as a sole trader. This involved registering with company house and filling out forms like for example articles of association and memorandum.
Before setting up anything, make sure you know what it entails. For example, if you set up as a sole trader and then take out a loan from the bank, you will have unlimited liability to pay back this loan as a sole trader, meaning that your personal assets such as your house will be at risk in order to pay the loan back.
If you set up as a limited liability company, then you have limited liability and risk only losing the money you have put into the company. This is a less risky set up but it subjected to tighter requirements in terms of making accounting information avaliable to the public for example.
There are many technicalities that you need to be aware of before setting up a company. If you're unsure, consult your lawyer, because you don't want to get yourself in the position of taking out debt as a sole trader with a reasonable prospect of being unable to repay it.
does that help a little bit?