I'm assuming you're year 12 going into year 23 in September? I kept pushing off my coursework but honestly don't underestimate how much time and effort it takes. Mine overall was around 100 pages, 15,000 words ... If you can, make your client someone you actually know, it makes it so much easier to document meetings, conjure up some feedback, outline requirements together etc.
Mine was my Dad lol, we just based mine on his main hobby which is hillwalking.
Don't be under the assumption that it has to be something like business related or that would be used in a real company, at the end of the day once it's done you don't ever need to see it again 😂
Do a good detailed analysis, don't worry about design too much initially. I found it so much easier to go back to design after development, because it was easier to write my psuedocode algorithms and make a variable list etc after developing. Only things I did for design before development was a problem decomposition diagram
Make sure it's something you can implement complex functionality into - actual processing needs to take place, but it doesn't have to look or perform super impressively or professionally.
I didn't know a programming language at all when starting mine and I got through it somehow and got a good mark, so I have every faith in you!