Live Life to the Best of it!
Year 18 is getting older and sometimes the best paths aren't always clear. Do your best to always respect those you don't know should be respected. Do your best to treat even the most wrong people how they want to be treated, often even if they are wrong, the wrong way could be right in their minds.
Also remember to live your full life, I'm still young myself, but I know that life goes way to fast for you to wait for it to happen. If you don't get out there and try things you haven't tried before or like to do, eventually you will loose the chance and it may not come back.
Don't let others control your life. Many will have views on how you should live your life, what you should say, how you should act, sometimes they are helpful views that you should let guide your path, but don't let them gain full control.
Talk to your family from time to time. This one may not apply to you as much as it might others, but your family won't always be there and you need to enjoy the time you have with them while you can. If you go off to collage or move away from your parents, make sure to call them from time to time, especially your mother. You have no idea how hard she has worked to give you everything you want or need, even if it isn't always the best way to handle things at the time.
Make good friends. Don't make friends with the cool kids just because you want to be cool, make friends with them because they are good people. Or make friends with the less liked people, sometimes they are just like you in ways you don't even realize. And don't abandon your slightly weird or open friends, often they are the best ones who will always stick beside you and usually the ones that you will keep forever. They are the ones who trust you enough to show you their best and worst sides. Don't be afraid to show them your wild side too.
There is probably a lot more I could add here or a lot that I am missing, hopefully this will help you though. It's your life.