Somehow I missed the part about forgetting the password.
Open iTunes on the computer. In the top right conner there will be a Sign In button (or your account name). If it says sign in, then you can click on it and get the following dialog.
If you click the Forgot Password button it will give you a couple of ways to get your password reset. If you are already signed in try to open your account by clicking on the account name. If you fail to enter the right password it will eventually give you a Forgot Password option in the dialog.
If I remember right (it's been a while since I had to do this for Apple) it will send a temporary password to your email address. You use that to log in and then change your password to something you'd prefer. There are several ways and places that you might end up needing your password. Most of them will offer this option, eventually if not right off.