How should I start my introduction?

The introduction is where you would be talking about why you're interested in the subject you have chosen to study. This could be a topic area you particularly enjoyed, or another experience that led to you wanting to study it. I would avoid the cliché words like 'passionate' - universities are sick of them. Give yourself credit where it's due and use a narrative style that is pleasant to read, but this overly floral type of language can be quite repellent and doesn't say much about you. Instead, try detailing things you did to further your knowledge in the area. Reading books, watching relevant TV shows and researching non-specification material are all excellent ways to show you genuinely love your subject, and show that you're capable of independent study, too.