So by now you've read the other entires saying 18 is largely an arbitrary number. This is correct. So socially, nothing 'has' to change. Just like you didn't magically transform on any of your other birthdays into an entirely different person, don't feel the pressure to now. Change comes from experiences - for the most part it happens on its own, all you have to do is decide how to react to those experiences, and this will affect how you change.
Legally, it's a different matter. Colleges/universities aren't required to share information with your parents unless you grant them the right. If you have a good relationship with your parents, I highly recommend granting them access to both your grades and your financial aid info - it's extremely helpful to be able to ask someone more experienced in the world questions about pay stubs from an on-campus job, for example. Additionally, again provided you have a good relationship with them, healthcare power of attorney. I hope nothing bad ever happens to you, or anyone else for that matter. But. If it does, now that you're 18 you're in a grey area of adulthood where technically the hospitals might not fully recognize your parents' authority. If, Heaven forbid, something happened to you, healthcare power of attorney grants your parents access to your medical information specifically in the case that you are deemed currently unable to make decisions for yourself. There are common-language versions available online; you can make statements on the paperwork regarding wishes (organ donation, do not resuscitate, no brain surgery, et cetera). Lastly, once you've got it all filled out, don't sign it yet. Sign it in front of a public notary & get it notarized. That way, if your parents ever need to pull it out, legally speaking the hospital won't even be standing on skim ice.
You'll also be at the age where you sign your own documents. Read them first. It's a good habit to get into. Keep a copy for your files. Keep pay stubs.
If you don't know how to cook, start helping your parents make dinner. College is a wonderful time to learn how to be an adult. Don't be afraid to ask people if you have questions, whether it's in class or about life. If it's in class, chances are at least three other people have the same question, nobody else has the courage to ask though.
Lastly but not least, "Good judgement comes from experience; experience comes from poor judgement". You will make mistakes. Learn from them. It's okay to be embarrassed, but don't beat yourself up over them. Do things you enjoy. Meet new people. Have fun. Life is a learning process. Nobody has it all figured out, not when they're twenty, not when they're ninety. Find your priorities. Decide which you're willing to compromise on, and which are set in stone. Listen when your friends give you advice, it's because they care about your success and you as a person. Always listen even if you don't intend to do something the same way someone else would. You're your own person, and that's a glorious thing to be. Don't be too concerned with how 'everyone else' sees you. The people who matter are the ones who've taken te time to get to know you. Don't stress over what you can't change. Sometimes things work out, sometimes they don't. Check the local weather before trips and plan accordingly. Take risks, as long as you're willing to live with the consequences. Take time to enjoy the small moments, it makes a difference.
One day isn't going to be the difference between a child and an adult. As with many things, it's more fluid than that. Some people are perfectly functioning adults and ninety-year-old children at heart. Some people are toddlers disguised as thirty-year-olds. Being mature is never the wrong answer. Remember to keep having fun, and stay curious. The world is full of things to be amazed by, if we only take the time.