Up until now the head of the household was responsible for registering anyone who lived at their address.
But now individuals can register by providing their name, address, date of birth and national insurance number.
The Electoral Commission warned people to avoid a "rip off" company that is offering to help customers for Â£29.95.
The move to a system of individual electoral registration is intended to reduce the danger of electoral fraud.
'Important step'
Jenny Watson, chair of the Electoral Commission, said it was a "huge step forward" in meeting people's needs.
"Previously we either had to telephone our local authority and ask them to send a form, or we could download a copy, fill it in and send it to the electoral registration officer," she said.
"The new system will be much more straightforward."
But Katie Ghose, of the Electoral Reform Society, warned the changeover, which affects England and Wales, needed to be managed properly if people were not to "drop off" the register and lose their right to vote.
"Today marks an important step forward in bringing our voting system into the 21st century," she said.
"But this is just the first step. If the transition to the new system is not managed well, and resources are not targeted at those most likely to drop off the register, then millions of citizens will go missing from our democracy."
Warning
A spokeswoman for the Electoral Commission also urged people to use the free registration service www.aboutmyvote.co.uk rather than a company offering to complete customers electoral roll registrations for Â£29.95 a time.
The watchdog has written to the company and requested Google remove its ads from its search engine.
"This so-called service is clearly ripping people off," she said.
"It is very simple to register to vote and we want to make sure no-one mistakenly uses this other service that charges people for something that is available completely free."
People can now register to vote via smartphones and tablets with the launch of a new free online electoral registration service.