One of the first things I found useful (and there used to be an intro getting started animation) was that to discover the features and functions of atom the one shortcut to remember is CTRL+SHIFT+P.
From there, have a browse when you have time, of the different functions.
Atom still behaves like a standard editor (CTRL+O to open, CTRL+S to save etc), so you should be able to start editing files straight away.
Another useful feature is syntax highlighting for the type of files you will be using. I mainly use C#/.NET and SQL files. Some language highlighting is included by default (e.g. SQL, XML). To set the language, either use CTRL+L or click the link at the bottom of the screen (“Plain Text” for a generic file).
Once you are familiar with the basics, I would suggest starting to browse packages. Open settings (CTRL+,), then click the packages tab on the left. There are so many to browse through. To find languages for highlighting, for example, type “languages” into package search and press enter. There are many packages for customising languages.
Then there are themes. In settings, under the themes tab, you can select them. For example, I use the default “Atom Dark” style, and “Monokai” syntax theme. You can browse through many different themes and select the one you prefer.
Hope it gives you some places to start. Enjoy