With SolidWorks, there are many ways to do the same thing. So you have your pick of what will be the best in the long term in how you create features--created for better editability, ease of use within an assembly, etc. These methods come with experience.
In general, I strongly recommend going through the built-in tutorials before getting frustrated with trying random things. The tutorials are good in getting you familiar with the basic tools. Also, the Help index will give you quick answers for most features so you don't need to wait for an answer from the forum. I'm self-taught as well, going back to 1997 when the tutorials had quite a few bugs, but I still found that to be the best way to learn.
For your current problem--
Pick each of the three planes on the left of your window--they should be called "FRONT", "TOP", and "RIGHT" or something like that. These depict your three primary planes that intersect your origen. Which of these planes is parallel to your flat surfaces of your cylinder? You can either pick a flat surface on your cylinder or a parallel plane, then Insert, Reference Geometry, Plane. You want to create a plane parallel to your selected plane at a given distance.
I recommend checking the Help file. Go to Help, and type in "plane" under the Index tab. You'll find all sorts of useful information that will help more than this post.
Good luck.
Jeff Mowry
DesignHaus Industrial Design
