Conventionally, you should begin every writing task from the introduction, proceeding to the body and ending with the conclusion.
Whether you write you pieces from an outline or directly by the seat of your pants, most people simply assume you're supposed to  write from top-to-bottom.
In practical applications, however, it may be smarter to write a piece in a less-than-linear way.
When I am tasked with writing about products I am unfamiliar with, I typically dive headlong into the details before I even think about an introduction.
 It's just easier that way, saving me more time, in the process.
The same is true for large writing jobs too, which probably requires you to write various sub-sections on a particular topic.
If one of the sections is just easier to turn out, why not start with it and let it kickstart your momentum? A good guideline is to start with the parts that you know best.
 Once that's done, you'll usually be deep enough into the material that you will also know how to write those parts you know least.
Some writers I've met, complete their pieces by writing the first phrase for each section.
Once that's done, they try to go through each one, trying to discover which one "writes itself first" then they go that way.
No matter what your high school writing teacher taught you, it's perfectly normal to write copy in whatever manner makes it easier for you.
Just remember to read it for coherence, revise what needs correcting and run it through a grammar software to ensure it adheres to proper English conventions.