Assuming you mean the cover:
The first step is finding a book cover template. You can find many online or through your word processing program. Once you have found a template, you will need to add your own images and text.
Here are some sites that offer free book cover templates:
https://www.bookcoverzone.com/
http://www.covervault.com/
https://www.template.net/design-templates/print/book-cover-template/
When adding images, be sure to use royalty-free or Creative Commons images. You can find these by doing a search on Google Images or another image search engine. Be sure to read the license before using an image.
For the text, you will need to come up with a title and author name. If you are self-publishing, you can use your own name. If you are creating a cover for a school assignment, you may need to use the name of the author of the book you are assigned to read.
Once you have added all the images and text, you will need to save the file. The file type will likely be a .jpg or .png. You can then upload the file to a site such as Amazon Kindle Direct Publishing or CreateSpace.