Your question is a bit too “open” to answer properly. Well, not really, but answering it would end up on repeating what others have already put in paper in the form of nice guides.
So I suggest reading through Macro Guide - Guides - Wowhead or a nice topic about the basics / some useful templates Useful Macro Templates - Guides / UI and Macro - World of Warcraft Forums (blizzard.com). Those cover quite a bit of the basics and are relatively easy to understand.
After that it may be a bit easier to point out what you want to know exactly or what parts are difficult to understand 