An abstract is a short summary of your own text (article, thesis etc.). In a condensed form it includes all the information necessary to understand what you have written, why you have done so, your position regarding the subject and in relation to its academic discourse.
If the work that you want to write an abstract for includes own research then you must also provide a brief introduction regarding its background, method, design and results.
Whatever your text may be about, writing the abstract should be the absolute final step. Yet be sure to not hesitate to write it, as having the ideas fresh in your mind can aid in the abstract writing process.
But before you do so, spare a moment to figure out who will be your target audience and how much they will be involved in the subject so that you will find both the right amount of information as well as the right language to convey it.