Assuming you have physical access you can boot into single user mode. In ubuntu, this is very conviniently called "recovery mode". If you lack this, go into edit mode in grub go to your user boot option and add the word single at the end
The 'proper' way to look up your username (and password, of sorts) is to look in /etc/shadow - cat /etc/shadow should spit out all that, and you'll see the usernames of all your accounts there.
You'll see something like
geek:$fhgbesft7y842387ty884yg87.:16026:0:99999:7::: - First 'field' before the colon is your username, then a hash of a password (I've replaced mine with key mashing) and so on. There's a more complete writeup here.
You will then be able to change your password with
passwd username as per your username on /etc/shadow