Well since you're already sorted on my first piece of advice (start small and let it grow) then my one piece of advice is:
when you make a mistake (because mistakes do get made, admit it immediately and immediately (24 hours absolute tops) do something even better to replace what went wrong. My business doubled in size and income when something went wrong and I came up with something better.
Don't overspend on your website. Be creative about how you have it made - use Shopify, Wordpress, freelancers on People Per Hour etc. Don't have your friend's brother's mate do it. It will suck and will cost you more than if you managed it yourself. Don't let anybody use what is called a custom CMS for your site. SHOPIFY. WORDPRESS. You have to be able to manage it yourself and whatever ANYBODY tells you, these are the cheapest ways.
Don't take on staff/premises until you absolutely have to.
However, find a good book keeper right from the start. You can use SageOne Accounts - www.sage.co.uk/sage-one-accounts - Sage is industry standard, and very easy to use yourself, and blends well with what a professional will use, so when you come to upgrade, the work you have done will smoothly go into Sage Instant Accounts or Sage 50 (the grown-up versions).
Manage your social media consistently. Better not to be on twitter at all than to have one tweet in 2011 and nothing since.
Be MEAN. You won't BELIEVE how quickly money will run through your fingers if you're not careful.
Any mistake you make (not talking about product delivery mistake) has already been made many times before and has cost businesses lots of money that they sometime have not recovered from. These mistakes are often very simple so you have started out very well by asking for advice. You can not EVER have too much advise. Finally, Never ever think you will not have enough stock. Too much stock is a business killer. Rarely does the value of your stock go up. 99% and more of the time excess stock goes down in value VERY quickly. As an example:- your supplier has an item that costs £1 each if you buy 10 but costs £0.75 if you buy 50 at a time. Unless you KNOW you need 50 do not be tempted. The difference between KNOWING you need 50 and THINKING you need 50 is failure.
My advice is firstly: know that you can't wait for sales to come to you, no matter what you are selling, be it product or services, you need to be tenacious but not pushy. What pushed me to starting my business ten years ago, alongside 5 part time jobs was the need to fund a course I was taking at the time. And it has been the very best decision I have ever made, I thought it would keep me going for six months, and then it turned into my career.