You can do a search on the internet for website backgrounds or something similar and find a number of sites offering them. Some for free, and some for pay. It is best, if you can find one you like, to use one that is smaller and designed for "tiling" so that it repeats. The keeps your site faster loading than images that are single and large.
The background is then added by going to appearance > background where you can upload it and set the different options to control how it looks and is aligned.