Hi
I need help on where to start in creating a webpage which users can buy items online.
What do I do first?

Take a breath. :)
I think you need to check on "shopping carts", one of the bigger ones that does this is PayPal. Im sure there are others, but Im not familiar with them.

If you aren't a web whizz, the easiest way is to go for some kind of hosted service. If you are in the US then RightCart (http://rightcart.com/) is meant to be a very good one.

Some other options are:
http://shopify.com/ - This one is free to run the shop but they earn their money by taking a 3% commission from the sales. Still well worth it though.
eBay shops/stores: http://stores.ebay.com/
If you just want T-shirt's etc. with your design try: http://www.cafepress.com

Do you know how to make a website? Do you have a Hosting account?

If Yes, and Yes, then your options are endless. You can go a full blown shopping cart (The simple one I like lately is www.ecommercetemplates.com) or go as simple as a simple paypal button (like this (http://www.stgabrielradio.com/2_su_mad.html)).

If the answer to either of the first two questions is No, then you've got lots to think about and do before you think about the tool you'll use to sell stuff.