Writing your personal statement can be challenging, so here is an example one for Economics. The key is to write it in three stages: Plan, write and review (probably sounds a bit obvious). To plan, mind-map topics that you find particularly interest in your subject and extra curricular activities you have done, which link to those topics of interest. Then form a thread through the different topics and super-curricular activities (i.e. things you have done beyond the classroom related to your subject) to form a linked and well-structured piece of writing.
Then the writing stage is where you draft your first paragraphs expanding on these points from the planning stage. You may have to do this several times, but don’t be afraid to do so. Eventually, after forming your first proper draft, you can move on to the review stage. This is where you get others to read your personal statement and where you read it several times, across different days (do not review in one sitting).
I have not elaborated too much on each of the three stages, because I feel that there are incredible resources already on the web and the writing stage is something that must be unique to you. However, this intro should set up how I wrote my personal statement and hopefully it will be helpful to you in this way.