Administrator Account

I bought and iMac Computer and had my account set up, so I thought as an administrator. If I had one, then I forgot the password. When I go into the Users and Groups, it only shows why Standard Account. I have gone through the instructions using Command +S and I never have a black screen pop up to enter any commands. When I use the Command+R and type in resetpassword, it says that it doesn't recognize command or command is unknown. how can I get me and administrator account or reset my password, if I had an administrator account, so that I can download software on my computer?