The Sith quest are very similar to the jedi quest first you have to be 40 job above to be able to change job. And Gather this item
15 Blue Crystal - Drop by Dark Poring - prt_fild01
10 Bears Footskin - Drop by Bigfoot - pay_fild09
After gathering these item use @warp yuno_fild02 193 315. You will see a black portal there named deep hole go inside of it and go straight look for anakin skywaker. Give him the item and he will ask you a question to become a sith. I don't know yet the answer never try doing sith but this guide is 100% sure. Hehe hope it help you. Thank you for all the comment.