A proof reader is someone who reads any form of written communication by an organisation or business before it is published or made available to staff.
To be a proofreader, you'll need to possess meticulous attention to detail and have a keen eye for potential mistakes. It will help if you have background knowledge of the industry in which the vacancy is being advertised because you would be able to spot factual errors as well as grammatical and spelling mistakes.
There are no specific qualifications required to be a proofreader but you might find that achievements in a relevant academic or professional field will be of great value in your application. For instance, journalists with the requisite NCTJ examinations under their belt would be looked upon favourably, as would anyone with an English Language or Linguistics A Levels or Honours Degree qualification. Membership of a professional body such as The Society of Editors and Proofreaders would be an advantage.
You may find online agencies willing to provide qualifications or training in proofreading and while this will do your job application no harm, it is unlikely to be viewed as strongly as copywriting, journalism or authoring experience. And experience really is key; if you can demonstrate a working knowledge of proofreading processes and the conventions of annotation that are used to highlight different types of errors, then you'll be in pole position in any job hunt.
Because of the nature of the work, proofreaders are often freelance contractors but rates can vary from £18 to as much as £30 per hour depending on the subject matter.