Don't use your real/full name.
Don't write down exactly where you live(rather just write down the city you live in).
Don't give out any personal details(such as exact birth date, age, parents names etc).
Don't share your photos publicly unless you know the person, personally.
Don't give out your mobile number/landline.
Don't let anyone know your password(s).
Make sure you have a good up to date antivirus.
If you have computer problems, go to the tech centre.
Don't visit a bunch of websites especially random/dodgy websites.
Don't be fooled by scams, spams, ads, dodgy emails etc.
Don't just click on anything.
Don't give out your email unless you trust the person.
Don't be telling anyone in the internet where you'll be, where you are, where you're going, how you'll get there, what you'll wear, who you'll be with etc.
Just avoid talking about and writing an essay about yourself, if you must then keep it sweet and short and don't get too detailed. Like instead of using your real name, use your nickname and make up a new name that rhymes with your name, instead of saying your exact date of birth(eg. 01/01/01) just use your birth month(eg. january), instead of your real age(eg. 20) lie about your age a bit by saying 19 and a half, never give out your parents names or siblings or best mates.