If you think that someone might know your Gmail password, or could easily guess it, you can change it. It might not be a bad idea to change your password once in a while anyway, just to keep a step ahead of anyone who might try to crack it. Hopefully, nobody would, but… it's a precaution worth keeping in mind.
How to Change your Gmail Password
Go to www.gmail.com and log in. When you get to your main "Inbox" screen, click the settings button () in the top-right corner, and select Settings.
Now, click Accounts and Import in the menu across the top, and then click Change Password.
You'll be taken to a new page with boxes that ask you to input your current password, your new password, and a copy of your new password (to verify it). Click in each of the respective boxes and type in the information requested (it will appear on the screen as little bullet points to protect it from prying eyes; this is normal).
Like it was when you first signed up for your Gmail account, your new password can be made up of any combinations of characters that you can type out with your keyboard, but it has to include at least 8. Try to make it something that you can remember, but is not too obvious. Gmail will help you with a little pop-up balloon that warns you if your password is too short, or if somebody could easily guess it.
When you've come up with a password that you like, write it down somewhere so that you can remember it. Then click on Change Password.
NOTE: Once you change your password, your old password will never work on your account again. With this in mind, it might be a good idea to keep track of your old passwords as well as your current one, so if you ever need to come up with a new password again, you won't have to worry about Gmail blocking one because you've already used it.