Do so at https://www.gov.uk/register-to-vote
You will need your UK passport number, National Insurance number and your last UK address you voted from, plus an email address - does also ask for a phone number in case they need to check any details with you as well. When you register it sends your application to the last voting office you had been registered at. As with any election, you can vote in person if in the UK, by proxy vote and postal (which I would not recommend, as 100% sure, it will not get here (only posted out a few days before the election I believe) and back to the UK in time!)
Whether it's worth voting or not, is down to individuals


