I teach technology education in a High School and frames are a project I often do with my students. If a ninth grader can complete a frame, anyone can.
Step 1: Its all about the stock
The best stock has a rabbet cut (the right angle cut) on one edge. You can also cut a rabbet with power tools, but having a rabbet edge is not 100% necessary. Hardware stores often have trim that can be used for framing. Craft stores and art stores often carry framing stock specifically for framing. However pieces with a rabbeted edge are not 100% necessary.
I use scrap wood, thin long scrap pieces are easy to come by on a construction site or lumber yard. No matter where you get it from you are looking for stock that is long and about 1" wide or wider.
The piece on the left is scrap wood. It was a 1x3, I shaped the rabbet and rounded the edge with a router.
The piece on the right was a scrap piece of tongue and groove flooring. The rabbet was made by cutting way the bottom edge of the groove, the tongue was cut away also. Scrap flooring is great for framing!