Gmail Account: How to change Password?

Hi,
i am a new user on Gmail and i didn't know how to change the password of my account. My account password has been leaked at some how and i am getting a huge trouble out. I have lots of me official documents as well as my private photographs including my family pictures which are there so i want it to make safe. Recently someone has stolen my pictures and he was trying to misuse it, If anyone with valuable information please let me know as soon as soon possible.