You are not as fat as you think. Do not compare yourself to girls in magazines or films, or even to your "skinniest, prettiest friends." Be yourself. Embrace the body you have; one day you'll be grateful for your curves.
Appreciate your friends whilst you have them, but do not become dependent on them. You will lose and gain friends over the years; each friend will teach you something new about yourself and leave you with memories that will last a lifetime, and you should be grateful for that.
Believe in yourself. You have the ability to go a lot further than you think and to achieve a lot more than you believe. Not everything will go your way but you will quite often surprise yourself with what you're capable of.
Don't mess about with boys. Wait for that one special person who can make you understand exactly what love is and why it's so precious, so passionate, so painful and yet such a vital part of life and who you are.
Love your family. They might wind you up constantly, but that's what families do! Never take anything your parents or siblings say as critical or mean; they are only trying to help and to protect you.
Do what makes you happy. You'll discover along the way that life isn't about conforming. It isn't about 'being normal' or doing what you think everybody wants you to do. It's about doing things you enjoy; things that make you happy, make you smile and make you feel good. Just because one person thinks something is right, it doesn't mean it is right for you.
And finally, trust yourself.