How do I know what to write in my personal statement?

You need to really show passion for your subject by saying all the things that you've done (eg. presentations, work experience, reading), but also make sure that it doesn't read like a list. You need to have a story running through it, or a set of themes (one for each paragraph) that tie together using a linking sentence at the start of the following paragraph. For example, a hypothetical situation: If applying for Biology, you might have done a practical marine biology course. You could start with that, and then highlight that you were so interested in 'x' aspect of the course (eg. the coral bleaching that you observed, when diving - this highlights knowledge of the subject and also your diving skills!), that you then went and read a book about how climate change and the increasing temperature of the oceans is causing coral bleaching. You could include a bit about the mechanism of this (corals expel the zooxanthellae living in their tissues and turn white) and why this is a problem (corals are under more stress and so are more likely to die). You were then inspired to take an online course about ways to combat climate change and the effects this might have on coral. Basically, take something you’ve done, and spin a story about how you developed a passion for it and demonstrated that passion, giving evidence of things you learnt and things you did to further your knowledge on that area (it doesn’t matter if you didn’t actually do things in that order - they’ll never know!).