If you want to start selling online, the first stage is to build a spreadsheet of your products and their details. The column headings to start with should be product number (SKU), product title, full description, net price, weight, product options, stock quantity, product category, sub type, etc, etc.
This spreadsheet and its derivatives will form the basis of csv files that will be imported into the eCommerce system's catalogue. There will be many additional fields that may or may not be needed, and the process will involve exporting, changing the spreadsheet, importing again, exporting again, changing, importing, etc.
The image in this post shows a sample product page in an ecommerce website design by Roxbourne - main product photo, title, SKU, description, price, optionals - some of which change the price, enlargeable thumbnails of extra photos. Click the image to see the actual web page.
Packaging and delivery costs need to be decided too. Will you offer free delivery? Optional express delivery? Most couriers have different rates for mainland Britain, N.Scotland, Ireland, Channel Islands - there isn't always one UK rate. So a delivery costs spreadsheet needs to be built to help work out the price bands.
What you do about shipping cost depends on many factors. Bare minimum, have a spreadsheet that includes product weights and dimensions and decide upon the geographical limit of delivery, what your terms are and who's going to process and despatch orders from your e-commerce website.
In addition to the product spreadsheet, product images need to be collated into category folders - from these the relevant sized web images can be created, web optimised and correctly named. You can't sell a product online without decent images.
As part of the product spreadsheet building process you'll have written enticing product descriptions. Now you need to write the content for all the other e-commerce web pages - the home page, contact us, about us, delivery terms, returns, contract terms, product category pages, help page, etc.