How can I change my password, without remembering my password from before?
While your question is decidedly vague (really, we could use the name of the service/software you're referring to) I decided to publish as the solution is fairly common and applies to lots of services.
If this is a web service like Facebook or Twitter, you will need to log out of your account and then click the "Forgot Password" link on the respective login screen. Here you can input the email address you signed up with, and a link will be emailed to you (not a password). Click this link, apply a new password and you should be able to login again.
If this IS your email address, you may need to dig a little deeper. Depending on the service, a secret question may be required before you can reset it. Other personal information like date of birth, or possible confirmation of your identity using saved mobile numbers is also often an option – it depends. Again, logout and click the "forgot password" link and follow the instructions.
If this is a computer, well that's a whole other ball game – we'd need the operating system at the very least. There exist tools that can break passwords, though generally these only really work on older Windows PCs (rarely on Mac or Linux systems).
Let us know if this is helpful, or if you have a computer you can't access, let us know what operating system it is running.