If you have not yet published your article and want to go Open Access, you have options.
Green Open Access is self-archiving a copy of your article, generally in a respository, as well as publishing in a toll access journal. Self-archiving before you publish in a toll access journal can (depending on the journal and their policies) hurt your chances of acceptance. If you want to wait until after you have been accepted, read After You've Published.
Gold Open Access is publishing your article open access. You can do this by publishing with an open access journal, or you can publish your article open access in a toll-access journal for a fee.
When you are shopping for a journal to publish your work in, remember to read the contract, read what rights you will retain to your work. See if you will be able to post it on your faculty webpage, or distribute it to your class, or classes at your institution. This is your work, why should you sign over the rights to it?
Even though you've already published your article in a toll access journal, you may still be able to make the article open access. Some journals allow you to publish a version of your article in repositories.
Check your author rights. What you are allowed to do with the article depends on the publisher. Some require you to wait a certain amount of time before you put your article into a repository. Some only allow you to put a pre-print of your article into a repository. There may also be restrictions on what sort of repositories you may use, such as restricting it to your institutional repository.