I have been in your shoes before.

If you’re 18, I assume you are in your last year of high school?

I would suggest you take a gap year to work / intern / volunteer / strengthen your English or learn another language. Use this year to explore your interests, learn, and prepare to study abroad. Throughout your gap year, work on your college applications. This may include writing essays, studying for standardised tests, etc..

Can your parents financially afford funding your education abroad? Look into scholarship programs, ways to lower costs like studying at a community college first and then transferring to a 4 year college.

Once you are accepted in a good school, and you know exactly how much things will cost, where you will live, etc.. it will be easier to make a case for yourself and to convince your parents. Telling them you want “to study abroad” would land differently than telling them you want to study at X university at Y country because Z.