A thesis is a big undertaking, usually 60,000 to 80,000 words. It’s likely to be the longest thing you’ve ever written, and when it’s complete, you’ll be examined on it. It’s very common to be worried about whether you can write a successful thesis. If you’re writing in English, and it isn’t your first language, that can be an even bigger worry.
The good news is that, if you get accepted for a PhD, it’s because your department believes in you. However, they don’t expect you to already have all the experience you need. You don’t have to already be familiar with the type of language expected in your thesis, or know how to structure it.