There are a number of online music theory resources available for free. Here are a few we can recommend.
Udemy offers a “How To Read Music For Busy People” course which can be taken for free.
Teoria.com has been offering free on-line theory instruction since 1997. It was developed by José Rodríguez Alvira as a complement for his courses at the Conservatory of Music of Puerto Rico.
EarMaster (which offers an online ear training software) offers a free introductory music theory course which starts with the very basics.
For those wanting to build on their theory knowledge, Stephen Wiles, a London-based music teacher has developed a series of music theory videos loosely based on the first five grades of the Associated Board (ABRSM) syllabus http://www.musictheoryvideos.com/