I have saggy skin and bad stretch marks below the belly button. What procedure would you recommend? (photos)

A tummy tuck will address the loose skin and stretch marks below the
belly button.  It will not remove the upper stretch marks but they will
be less noticeable.  Since you are only
20, keep in mind that any additional pregnancy can have an effect on your post-surgical
result so I would suggest waiting until after you are done having children before
moving forward with surgery. Seek a consultation with a Board Certified Plastic
surgeon to discuss all of your options.

Your story and photos suggest you would benefit from a tummy tuck.  It is the only way to remove the stretch arks and loose skin.  Since you are 20 you might want to wait until you are sure you are done having children.  Good luck

   A tummy tuck would probably be a reasonable surgical alternative if you have finished having children.Kenneth Hughes, MDLos Angeles, CA

We can remove the lower stretch marks with a Tummy tuck but the upper ones are difficult to get rid of. You should get a consultation with a board certified surgeon and dicuss the options of stretch removal. 

Exercise works for weight loss but you can't "spot reduce" or tone skin. If there was a muscle separation (called a diastasis) then that cannot be corrected with exercise either. That is why we do tummy tucks instead of sending patients to the gym. From your description and the photos, a full tummy tuck is likely to be your best option. See a board certified plastic surgeon, preferably an ASAPS member with a focus on aesthetic (cosmetic) plastic surgery.