You can consult your agreement, the publisher’s website, or SHERPA/RoMEO, a searchable database of many publishers’ open access and institutional repository policies. Most major publishers will allow you to place a version of your article into an institutional repository, but many of those request that you not use the publisher’s version, and instead post a pre-print or post-print manuscript (see below).
If you have not yet signed a publication agreement, we encourage you to find out your publisher/journal’s self-archiving policy before signing. Some publishers already have self-archiving permission as a standard feature of their agreements. If yours doesn’t, or is too restrictive, consider adding a standard publication addendum, with which many publishers are already familiar. SPARC (the Scholarly Publishing & Academic Resources Coalition) has a short write-up on copyright management and your rights as an author. Their standard addendum is downloadable from that page. Additional addenda for authors, made available from different organizations, can be found here.
We are also, of course, available for consultation.