Under U.S. copyright law, as soon as a work is fixed in a tangible form, you as the author immediately have certain exclusive copyrights:
When your paper is accepted for publication, many publishers will ask you to sign a standard publishing agreement which transfers all copyrights in the work to the publisher. That may not be necessary, nor to your advantage.
If you decide to keep your copyright:
1. Register it. While you do not need to register your copyright in order to maintain your rights, registering your copyright can be useful. It allows you to claim damages in case your rights are infringed, and it lets people know where the copyright resides if they wish to ask permission to make use of your work.
2. Consider including a statement that pre-approves certain uses of your work. Creative Commons provides a suite of standard licenses which allow you to automatically grant certain permissions for the reuse of your work. A guide to Creative Commons is available on the Library's web site.