Amazon webtore and pro merchant account

I need help to set up the amazon pro merchant account and webstore account. I would like to trial selling flower bq on the site with around 15 products but am already finding it hard to understand how to upload new products as Amazon seems mainly set up for existing product ranges. I have started the spreadsheet but the image upload requests a url and all my images are on my computer so unsure how to upload. I thought the webstore would be a good plan and have registered but unsure what to do now. I am sure going on Amazon is a good plan but need help and advice in setting up and also maximising the site effectively.