It's not gonna feel like that. Literally nothing changes, you're gonna do the same thing you do every day, whatever that is. 18 may be the legal age for adults, but for most countries, you still can't drink or do most of the things you can't do by 19/21/whatever the age is in your country (maybe it's 18 where you live, i dunno, it's 19 here). Even when you do become legal age for things like drinking and stuff, nothing actually changes. Your life will continue on as normal. There's no special ceremony that happens, there's no MaGiCaL TrAnSfOrMaTiOn, you just happen to be one year older. I'm 19, I'm supposed to be an adult, but I still sit around drinking Pepsi and watching Family Guy and drawing all day long, it's the same shit I've been doing for the past like, 2 years? Yeah.
"Adults" are just children with more privileges, responsibilities, and life experience. At the end of the day, it's still just a word in the dictionary that we use to segregate expected maturity levels.
Congrats on the 18th birthday though u w u Hope you have some fun way to celebrate!