It's simple, look up "Atom Software Download" and then go on the link, click "Download" and install! Or, if you're really lazy, there is a download button just below this. Click on the link and it should start downloading, unzip, and begin!
Atom is one of the most comfortable softwares, fitting to almost every need. If you need, go around in the settings and try out all of the settings! When you want to start, go into file, and open a new file. In the bottom right, where it says "Plain Text" you can change it to any code language. You can also change the line endings and encoding. One extremely useful thing you can do with Atom, is start new Repositories through Github. This will probably be easier to understand the Github repos if you understand Github.