Will septoplasty cure my sleep apnea? Will I be able to use my bi-pap machine immediately post surgery?

A septoplasty alone will not have much of an effect on sleep apnea. That being said it often makes the CPAP or BIPAP machine more tolerable and easier to use. Typically, most surgeons want you to wait two weeks before using the machine but I would ask your surgeon on his/her preference.

Septoplasty alone is not normally an adequate treatment for sleep apnea. If you just have a septoplasty, without any manipulation to the nasal bones it would likely be okay to use your BiPap. 
I would consult with a plastic surgeon and make sure to ask these questions. Bring your BiPap mask into the office so they can see where the pressure points are. 
Hope this helps.