If you haven't worked with computer code before, it's useful to cover some of the fundamentals before beginning your study in September. If you already know the basics, you'll find it easier to progress quickly once you arrive.
The first two online courses outline the Python programming language that you'll be using a lot. This is a really versatile language and you'll be able to put it to work for lots of things. And happily it's pretty easy to get started with.
Then there are two courses on web development: one on HTML which is used to build web pages, and another on CSS which often controls how they look.
Finally there are two more online courses. One is a series of challenges to work on your Python skills and solve some problems; the other is an introduction to Javascript, another powerful programming language you'll use from September.
You don't have to use these courses; if you want to learn the same stuff in a different way, go for it.
Intro to Python for data science (Datacamp)
Learn HTML by building a cat photo app (Free code camp)
Learn basic CSS by building a cafe menu (Free code camp)