How do I get dual citizenship? Both my parents live here; both have SL passports; I have a German passport; I was born here; and I have a birth certificate from here.

i dont know the process I'm in Colombo right now. Can anyone assist me on how to do this? There are so many immigration agents who want to help, but they do ask for a lot of money.
How long does it take to get it and how much does it cost? I was told I had to deposit Rs. 30 lakh.