The informative Society of Archivists website is the place to start.
There is a postgraduate qualification in Archives and Record Management, available full time or part-time. It may be necessary to undertake volunteer work experience in order to get on a course, as competition for places is stiff - the Society can help with placements.
About half of archivists are employed in the public sector. Other employers include museums, universities, national archives, businesses, courts and charities. Most jobs are advertised through the Society's recruitment publication.
A structured entry such as this does have attractions over the vagaries of entering a field without any qualifications necessary to do so, i.e. where the applicant is reliant on merely endearing oneself to an employer over the next person.
You are unlikely to make a million it must be said.