This article from The Poetry Foundation provides a nice, brief overview of Johnson's life and work. It includes some biographical information about her early life as well as a list of journals and anthologies that published her work.
This article is published by the African-American Registry, a nonprofit organization. It includes some of the same biographical information that can be found about Johnson on other sites; however, there are some interesting details about her work, particularly about other Harlem Renaissance figures that influenced her career.
This New York Times obituary for Johnson was one of the most thorough biographies I could find online, It was one of the few sources to explain why she stopped writing after her marriage, and it went into depth on the type of "transitional" poetry that she wrote. It closed by including one of her poems.
The website PoemHunter compiles poems by various authors. It had two of Johnson's poems, "Ah my race" and "The Sandman." It was helpful to read the poems themselves so that I could see Johnson's style firsthand, which was described in several of the sources I read.
This was a very helpful source. One reason is that it included my favorite poem of Johnson's, "Invocation," along with a brief analysis of the poem. It also had a few biographical details that other sources did not include. It was published by The Washington Post.