There are sites claiming to assist in registration which simply data mine the info you provide. If you don't know where to register online, Vote.org contains the links for every state (they don't require your information to access the links).

You can also simply go directly to your state's official Secretary of State website, which is what Vote.org provides, and register that way. Please keep your data safe and avoid inputting personal information on any sites other than your state's official Secretary of State site.