Hi Leonora,
Welcome :) First of all, the best place to get tons of information about starting the shop is on the Etsy Seller Handbook. There are tons of articles on how to format things, how to set up your listings, etc. Reading through the team discussions will give you lots of ideas and information too.
Etsy makes a lot of this stuff really easy for us, so it's not so much trying to design a website as it is designing a shop that happens to be online. If you were to open a storefront, what would you want it to look like? How would you present your work? What would draw your buyers in? Those are the things you should think about when designing your banner and your listings.
Your banner, like Harshey said, is like a sign for your shop, to draw people in and make your shop look cohesive. One tip from the handbook is to use a photo of one of your items as a banner, then add some text to it. Take a look around at different shops on Etsy to get an idea of what banners could look like, and then start to design yours. Mine has evolved several times, and you can change it to go along with the shop as it evolves and takes its full form.
Initially your shop will probably be a little scattered. You want to try to evolve it to look like a single store. Have your photos all taken on backgrounds that are complementary to each other so all of the items look like they belong together. Having a cohesive shop brings people in, and keeps them looking at all of your items.
As for making your banner and your photos, there are lots of tips on Etsy's handbook pages for you to review. If you end up having specific questions, you could always ask the team and people will pitch in where they have experience.
But before you go putting up your listings, I would suggest getting a banner up that you like, finalizing your policies and your shop announcements sections, and taking tons of product photos to select which ones you like the most.
Hope this helps!