Although you mention you are not computer literate at least you want your computer to be safe and in order.
--- I do not have Windows 8 so all I can do is provide some ideas
--- However I recommend you run your security programs first before moving on
Following is a summary of your concerns and questions
--- Navigating through the event viewer is easy but understanding it is proving difficult
--- Find out which programs and files were accessed
--- Was anything connected to another computer on your network?
--- You haven't erased anything so all of the data should still be there
Although your question focuses on using Even Viewer I think it would be a good idea to work out a couple of simpler items first that I suspect you don’t need Event Viewer for especially since it will take some time to get to understand how to use Event Viewer
Since you haven't erased anything, the data within the time frame you are interested in should still be there
On finding out which programs and files were accessed
--- Check out Microsoft: How To Search On Windows 8 – File Search, Settings Search, App Search http://windows7themes.net/microsoft-how-to-search-on-windows-8-file-search-settings-search-app-search.html
--- If you a file you do not recognize, right click and select Properties. Look at its origination & last accessed date.
--- Go back to the list and see if you can open that file.
------ If it opens see if you can relate to it
------ If it doesn’t open, it could be a file that was used by that person from whatever source they used, probably a thumb drive, maybe a CD. You will have to evaluate it according to how you perceive it
We can move on from there after you check this out