Nice question Shazia. When you post your paper online somewhere outside a paywall, you join the majority of scholars and your paper will gain a citation advantage over papers available only through a journal. This has been demonstrated in recent research. A particular focus of research on open access publishing has been the hypothesis that open access papers will be more highly cited.
Open access journals can either be free or paid journals. Excepting a few highly reputed international journals, it is not advisable to publish in paid journals. Publishing in open access journals increases the visibility of research and a higher probability of citations. Some reputed publishing firms offer high quality publishing opportunity with a fee. If it is affordable for the authors, they go ahead with publishing their research in those journals with the hope that it will increase the chances of citations.