Elaborating on "hmccrory"'s answer:
You want people to read your editorial, and afterwards to say "that's the best I've ever read. Hire that person!"
So, start with a snappy title, something like "Solar Garden Light: The Ideal Way to Grow your Garden". This gives the possibly interested reader a reason to read further.
Start with a sentence or paragraph that summarizes what you are going to say. Be sure to write clearly and interestingly. One way of attracting interest is to make the article personal: "Last week I was working in my garden ..." or "This year my garden is beautiful; last year it was a mess of dead plants".
You should already have a list of the points you want to make, which are the most important (you'll write about them first), and how you want to make them. Include evidence if needed, and it's a good idea to include negative evidence as well, as long as you still justify your point. This keeps your editorial from sounding one-sided: "All my ideas are wonderful, and nobody else counts!"
And you might be a little casual. If you've got room you could say "I did this wrong, silly me". Or "My mother told me this would happen".
If you have fun writing it, it's likely to be fun to read as well. And be sure to have somebody else read it before you hand it in, in case they say "what the heck are you talking about?"
The webpage below would be good to read.