Nobody enjoys writing their personal statement. The first reason for this is that it is very difficult to find a middle-ground between brashness and dullness, cliches and unusual innovations. The best way of doing this, although it may be embarrassing, is to seek out feedback from teachers, tutors, parents, and friends, and to make revisions where appropriate.
The second reason it is so hard is that there is no one right way to do it: for better or for worse, your personal statement will end up being a reflection of who you are as an individual and an applicant. Having said this, there are 3 areas which a medical personal statement should usually touch upon:
What draws you to medicine. There is probably no truly satisfactory way of describing that without touching on both the intellectual and the social sides of the profession (for example, a love of organic chemistry and a sense of vocation for public service). You will need to answer this question at interview so it is important to start thinking about it anyway.
What you have done to get you towards a medical career. Admissions officers want to find out about positive steps which applicants have made towards entering a medical degree, outside of academic life. This could include anything from volunteering at a nursing home to visiting a medical exhibition at a museum. The only essential thing is that they want to know that you have thought deeply about what you have done, what you have taken from it and how it relates to medicine.
What you like to do outside of medicine. Admissions officers genuinely think that well-rounded individuals make better students and better doctors! This does not mean you have to have a gold Duke of Edinburgh Award and play squash for your county in order to get a place at medical school, although ideally you should be able to point to activities that involve teamwork, and a certain breadth of interests. But whatever the specifics, you are unlikely to go far wrong so long as you think about what it says about you and how it might relate to medicine.
A reasonable starting point for the first draft of your personal statement might be to split the word limit into 3, and respond to each of these areas in turn. Of course, this is just a template: for instance, certain medical schools, particularly Oxbridge, might prefer a more academic focus, and if you have done anything especially interesting than it might make up the bulk of your statement.