Everybody needs to register themselves. It's straightforward and quick and you can register online at www.gov.uk/register-to-vote.
If you are unable to register yourself, it's ok to get help filling in the details, but you must make the declaration yourself. In certain circumstances, someone who has been granted appropriate power of attorney may be able to make the declaration on your behalf. You will need to speak to the elections team to see whether this applies to you.