Sam,
The trick will be, to not destroy the "original" picture especially while you learn!
Using "Paint" that comes with windows, not sure if win8 has it though, if that pertains to you....
Copy the picture from where ever (NOT MOVE) to a folder. this way you can always start over for one thing, plus you have the original. In the folder called my pictures, I have a folder I actually named "edit" where I carefully drag and drop (making sure to "COPY" as I do this, by holding down on at least one the Control Keys (labeled CTRL, located under both shift keys) as I click and drag... while you are there rename the file, so when you get done and move it back, it wont over-write the original.
once you get it in there, open Paint, draw your lines do whatever, then save, if you didnt change the name from example "dsc0002.jpg" that came from the folder, do "save as" instead, which will give you the opportunity to change the file name. Doing this, is just like making a photo copy of something before you modify it, you know. leaving your original to be kept safe.
hope that helps a little.