e-commerce store put on my preexisting site

I have a pre-existing website that we added a page to for a e-commerce store. All shopping cart software is already running but nothing on the page within the website. We have a cart, but no pictures or accessibility on the site itself. I would need someone to basically build the visual and functional store inside the site itself and link it to the cart. All credit card processing software and gateway are already established, due to this originally being a paid membership site. Now we want to add store. We would like to have approx 4 categories to organize items and 2-20 products per category. Thank you.