The summer before your final year of high school is an important time to get started on your college applications.
Read. The news, fiction, memoirs, short stories, essays. Being a good writer is all about reading. Check if the colleges you are applying to have a recommended reading list.
Finalise your college list. Pick 5 for the UK and around 7-10 for the US. For both, get a good spread of reach, likely and match schools.
Organise your work. Input all required information and deadlines on a spreadsheet. Work backwards. Get as much done as possible before school starts again.
Secure your recommendation letters. If you haven’t done this, email your teachers. Arrange to chat with them to let them know what you’ve been doing outside of school.
Just write. Even if it’s bad - you will cross the first hurdle by getting started. Have a draft ready by the time school starts.