Employers don't generally ask for your educational or vocational qualifications. You will benefit from good levels of general education – at least degree in any subject. You can go for training programs for indexers. The Society of Indexers (SI) offers different training programs for the indexers. Training in Indexing distance learning course by SI is comprised of four units, each involving 45-50 hours of study. After passing the exams after each unit, you can get an Accredited Indexer status, which is recognised by most big-time employers. You can also opt for further qualifications by taking SI second stage professional qualifications.

Academic qualifications are not the biggest thing, but your understanding of the topic you are indexing is. By joining professional bodies like SI, you can get plenty of networking opportunities.