Have or take the G.E.D. (General Equivalency Degree). Most colleges accept this in lieu of a high school diploma. Guidelines vary from state to state, so be sure to check your state's website for more information.
Contact a recruiter at the online school you're interested in, and ask about dual enrollment possibilities. This is also called concurrent enrollment. This will allow you to take courses for your online degree while finishing your high school diploma at the same time.
Apply to online colleges which do not require a high school diploma. According to a 2006 "New York Times" article (http://www.nytimes.com/2006/05/30/education/30dropouts.html), almost 400,000 college students in the U.S. do not have a high school diploma.
Contact a recruiter or guidance counselor, and ask about any options they have for adult students without a high school diploma. You may qualify as a non-traditional student and thus be able having to have a diploma to attend.