The roku account isn't about buying anything, and it isn't required that you purchase anything, but a valid roku account is required to use the device. It has to be linked to your roku account as that is how you get different apps/channels available on the device.
If you have an existing account, and know (and still have access to) your email address, then it is a simple matter of resetting the roku account password,
which you'll do online from your computer. Click here to do that: https://my.roku.com/password/reset
If you don't know what it was under then just go ahead and create a new roku account under your current email address - as long as there not an account already under your current email, it will go through just fine. This is simply a registration step to authorize the roku device and let you start using your apps.
If it won't let you, then you're using the wrong password, and would have to reset it via the link I sent.
If it didn't go through (you have to check your email, they will send you a confirmation email, you have to click on a link in it, which opens a web page, and prompts you to choose a new password), check your junk/spam folders.
Remember, you can always create a new account if that is easier (if you have another email address you can use), there is no cost to a roku account, unless you opt to purchase paid apps through it, which is not necessary.