Once you fulfill the legal requirements for your state to
your own satisfaction, you can "award" yourself a diploma. It won't be an official, accredited diploma, but that won't matter so long as you can back it up with a portfolio of work and/or nationally standardized test scores. CLEP, AP, and DSST are all accepted by high schools and colleges as proof of completion of the corresponding high school-level material, and most colleges accept them for first year college credit as well.
Basically, an accredited diploma gives you a pass out of high school with a single piece of paper. A non-accredited diploma has to be backed up with portfolio and/or test scores. But they're both accepted by colleges, especially now that colleges have discovered homeschoolers make better students.
Many homeschool groups organize a yearly graduation ceremony / prom if you're interested in that sort of thing, and can print you up an official-looking diploma.
Check your local homeschool groups for info.