there are quite a few options to choose from. the main ones are: 1- you can delete the account and then just make a brand new one. 2-you can change your password on the account. 3-you can also notify the email provider. make sure you never share your password with anyone.
You need to change your password and make it something only you would know. Many times people include in their passwords a number or special character in order to make it more difficult to figure out. Special characters would include the alternate symbols on the number keys of your keyboard. Unauthorized e-mail access is very scary. Hope this helps!
I would contact my internet provider and let them know what happened. They can do a search and track down where the person that accessed your mail's computer is at and find out who they are. You will proably have to close that account and get a new one.
You should change your password right away, and consider getting an entirely new account if at all possible. You should also notify not only everyone in your address book, but also the system administrator for the e-mail service that you are using.
Personally, the first thing that I think you need to do is change your password. Then contact whom ever got into your email and let them know that you plan on turning them into the police. Also, make sure that your personal information was not stored on your email like your credit cards, ect.