I have hard time trusting people and being friends with them. I'm afraid of getting hurt. At school I tried to socialise but the bad memories kept flashing back in my mind. I don't know what to do. I really want to open up myself. How can I?

By socialising and taking risks, life is very much about the latter because you have no foreknowledge about what is going to happen. There are many good people out there who feel just the same as you.  Look into the obstacles that get in your way which may have contributed to such experiences, ie inability to say No, wanting to be liked so acting at your own detriment (people pleasing), not listening to your gut reaction, over disclosing yourself to early in the friendship, not asking yourself if you really like the person etc. At least this way you will be able to recognise any of these thoughts or behaviours if they reoccur. Maybe try doing stuff together with new friends, because group activity bonds and avoids needless gossip and oneupmanship. You just have to go for it and remember life is full of lessons often painful, but don't allow that to stop you from having fun. When the memories come back just put them aside and say "Thanks for sharing but I'm ok, this is not then, this is now." 

Also try doing new things by yourself, things you enjoy, because if you build a strong bond with yourself then you will be more attuned to what you will and will not tolerate in a friendship. Place your happiness in yourself first, not directly in others, or else you will become codependent on them for how you feel, making you easy to manipulate. Some people do not let having no friend to go with stop them from doing what they want. Independence is always a an attractive trait, it shows strength of character, they enjoy and trust their own company.