Publishing your work Open Access allows greater visability of your work by giving everyone access to your paper regardless of whether or not they have a subscription to the journal. There are two routes to publish OA, green and gold.
Green OA allows you to publish without paying a fee but then deposit your paper in a repository (usually after a short embargo period) such as research gate or an institutional repository such as one managed by the university you may be affiliated with. To find out which journals support Green Open Access publishing, search Sherpa/Romeo by journal title.
Gold OA requires a fee to be paid, known as an Article Processing Charge (APC) where the paper is then made freely available on the publisher/journal website. You can publish Gold OA in either fully OA or hybrid journals. Fully OA journals are funded purely by APC's, hybrid journals are funded by both traditional subscriptions and APC's. To find out which journals support Gold OA publishing and how much APC's are, go to the publishers website and search Open Access.