Try not to let CSS intimidate you, I understand it can be pretty overwhelming getting started/coming back after a long period of time. A good plan is probably first to get the basics of linking up a stylesheet understood. And then from there understanding how this effects your html. Have a read of this:
Once you've followed through some of the above tutorials, working with a background image is probably actually a good way to get your feet wet with CSS. Mainly because (if you do it the right way) it will introduce you to different selectors and some browser compatibility prefixes. Anyway, take a look at the following CSS:
background: url(img/background.jpg) no-repeat center center fixed;
The first thing I will point out is that the second line, in which you declare your intended background image would in and of itself suffice to create a background image. However the above CSS is a good way of ensuring the background image will cover the entire browser window, dealing with different screen sizes, devices etc which is half the battle when it comes to building websites these days.
Secondly, you will notice we are applying this style to the html element, rather than the body element, which may have been what you expected. The reason for this is that the body element doesn't immediately take up the entire height of the viewport whereas the html element does.
To specify which image you want to use, simply declare a background: url() and paste in the directory path to your desired image within the parenthesis. If your stylesheet isn't in the root folder in relation to your images, you can begin the path with ./ to return to the root directory. Then we apply some values to our background property. Firstly, no-repeat, this ensures our image isn't repeated in a tile like fashion. Next we position the background image. By default it will start at 0, 0 (the top left). Since we are intending to cover the entire screen we need it positioned in the middle, so we specify center center to ensure it is placed 50% horizontally and vertically.
Finally we specify the background-size: cover; The background-size property in CSS is really useful, but also a bit complex. If you feel like it, read up on it a bit. But for this I'll suffice to say specifying cover ensures the image will stretch to fill the container whilst maintaining width/height aspect ratio. You'll notice we've declared this four times, with different prefixes at the start. We do this to ensure different browsers support our cover. Webkit covers chrome, Android, iOS and safari I believe. moz covers Firefox and o covers opera. I believe microsoft browsers support it directly, but if not we can use the -ms- prefix.
I hope some of this will be helpful, as I said don't let yourself become overwhelmed with the amount of things there is to learn, these things take time and no one ever stops learning and improving. If it's too much just grab the first line of code and it will suffice to begin with.