Install an antivirus/firewall:
First of all, you should have an antivirus active to scan all the applications you install for malwares and threats. Everything you download is dangerous unless your antivirus proves that it’s safe. Hackers today tend to spread those spywares and hacking soft wares that collects all your data and private information and send them to them.
Really, you should be the only user allowed to administrator’s access .Because other users with those access might access to your files easily. But I prefer restricting access to your computer, because nowadays there is some software that enables anyone to log in into your pc and into your user without any password.
Hide your sensitive information
Hackers can use any information available about you to hack you; either it’s your birthday, your school name or anything. So as a start go to your privacy on Facebook, and hide your profile from the people that you don’t trust, and be careful who to add, and what to share. And don’t register in sites that you don’t trust, or sites that asks you for your info. You can simply create a second email ( on Yahoo/Gmail/or msn) for those sites that you don’t trust .
You may also use proxy and encryption software to prevent sharing your location and information. But remember what you don’t share they’ll never know.