If you want to share your work with an open license, you must be the copyright owner. Authors and creators own the copyright in their works by default, unless some or all of those rights are transferred through a license. Many publishers request a full copyright transfer, but authors do have options other than giving up their copyright ownership entirely.
Many open access publications are made available under Creative Commons licenses. Creative Commons gives you the legal tools you need to grant others permission to use your work under the conditions of your choice.
Creative Commons licenses do not replace copyright but work alongside it, allowing you to keep your copyrights while permitting others to make certain uses of your work. You can choose to permit or deny commercial (for profit) uses of your work and the modification of your work to create new works, and whether or not any new works have to be licensed under the same terms that you chose.
Once you have learned more about the licenses Creative Commons offers, you can select the license you want for each of your works. Using the license chooser tool, you can generate code that you can easily embed into a web page. Creative Commons also has tutorials for marking your works with a license notice in a variety of other formats.