If it is a cover for Word, the best program to use is Word.
You need to take into account that if you are preparing a Word document to be delivered that document maintains the editability. So the cover itself should maintain that.
But Word can incorporate a background, and that is a specific element that you then need to combine with Word itself.
For your cover, you can look for a free background that suits your content. Perhaps a photo or an abstract image.
And for the styles themselves, look for a style template. There are a lot of them for you to find.
It is very important to remember to use styles, and not add format to the paragraph directly.
Design is a profession, it is not an easy task that can be learned just in one week or in a post or tutorial. So to answer the question "how to design" is almost as asking for you to switch careers.
A good option is to find an example of a document you like and try to mimic that document using Word styles.