This guide will help you get started if you don't know much about coding.
A big mistake many newbies make is using the Visual Editor. If you want to code, do NOT click the 'Edit' button. Instead click the triangle next to the edit button, and click "Classic Editor." Then, switch to source mode using the tabs on the top. This is where you code. The Visual Editor does have a source mode option, but it will sometimes screw that up and display the code and not the effect of the coding. Save yourself the hair-wrenching frustration of coding properly but seeing your coding displayed as regular text and use the Classic Editor.