I'm in the middle of writing a crime/thriller book on Microsoft Word 2010. But after writing it, do I have to make a front page cover? Or does the publisher do it for you?
Unless you are a major bestselling author – such as J. K. Rowling or the president of the US – and have the power to take your cash-cow-book to another publisher if the present one does not comply with your wishes, the publisher will decide the cover. Some publishers allow their authors to have a word in cover design, but many don't – and most intelligent authors do not try to meddle in what they don't understand.
The cover is probably the single most important factor in book sales, and no professional publisher would allow an amateur such as an author to decide such a critical success factor. Big publishers have a marketing and an art department, smaller publishers hire experts, who come up with a marketing strategy for a book and a fitting design concept for the book cover. When these have been discussed with the editorial and sales department (and maybe the author), a designer (or painter) is hired to execute the idea. An art director will oversee the design process. The designer will suggest a handfull of layouts to the art director who will chose one and ask for certain changes. Then the designer creates the cover. This will then be presented to the marketing, sales, and editorial departments, who will usually agree to use it, but sometimes hire another designer to create a better one.
I worked in the PR department of a big publishing house and witnessed this process first hand. Different publishers will do some things differently, but the basic procedure will be similar to what I have described.
If you have a publisher, you can leave it to them. And as @user5645 stated, you probably won't get a choice.
For me, I choose to design my own covers. (I don't have a publisher, so that's why) If you chose to take the path that I've chosen, I would suggest doing some research. Research what makes a good cover, what sells the best, what fits the genre, etc. It's like researching video games to be able to make your own: you have to research what makes it better.
Now, I haven't published a book yet (I am currently writing my first), but it just takes time, practice, effort, and research.