You can design your own cover which I did for my first book or you can have an outsider design your cover. You may also ask your publisher to design one for you. Once the front cover is designed be sure if you had someone do this other than yourself give them credit for it, discuss payment, and get it in writing before you go to print what the arrangement will be.
Looking back to my first writing experience it was scary and overwhelming. Today, I am working on two at a time. So, just write. Just do it by faith. Do what you can do. God will meet you. He met me, and He is no respecter of persons.