Whether it is a book cover, poster, or something as simple as a desktop background, there are certain steps you should take when designing things.
1. Use a decent photo editing program
What I mean by this is, don't try to make anything like a book cover in Microsoft Paint. It will just look bad and unprofessional. I would recommend using Photoshop, or it's free open source alternative GIMP. I have been using GIMP for the past four years and love it, it is very user friendly.
2. Be Creative
This may sound very cliche, but do not limit yourself to what you can do when using GIMP or Photoshop. This is what I struggle with the most. Step outside your comfort zone, if you try something and it looks bad, retry until you master it.
3. Use your own images, or royalty free images
If you are working on a design that you plan to publish or sell, make sure it is your own picture, or a royalty free picture. Royalty free pictures are licensed to users under CC (Creative Commons). Any of these images can be used and edited/sold for profit. By doing one of these two things you won't have to worry about infringing copyrights.
4. Use good fonts
Most of the time you will have to import fonts into the program via downloading them and inserting them into the program files. I will make a tutorial on that in later weeks. Fonts can make or break a picture, I would not recommend using Times New Roman for anything other than actual writing. Never use Comic Sans for anything, please! Under no circumstance will it look good on a design. One of my personal favorite fonts is Marlboro, it is similar to Times New Roman however it is much more presentable on a picture.
These are just basic things to keep in mind before you create something, there will be more in depth tutorials on GIMP to come.