The Verdict: Clear Path
You have been Indonesian since birth, because a parent was Indonesian when you were born on or after 1 August 2006. If you also hold another nationality, you must choose one of them by the age of 21.
Since the Citizenship Law of 2006 (Law 12 of 2006) took effect on 1 August 2006, a child is Indonesian at birth if either parent is Indonesian, wherever the child is born. Where that leaves the child with two nationalities, Indonesia allows it only as an exception for children.
What to do now: register your birth with the Indonesian civil registry or an Indonesian embassy or consulate, using your Indonesian parent's identity documents and your birth certificate. You can then hold an Indonesian passport. A child who holds two nationalities can also use a facility that lets them enter and leave Indonesia on the foreign passport.
The deadline: within three years of turning 18 (or of marrying, if earlier), you must declare in writing which nationality you keep. If you keep the other one, or say nothing by 21, you stop being Indonesian. A former dual national can still get a permanent stay permit in Indonesia that lets them work, so choosing the other nationality does not close the door to living there.
If your parents were not married and only your father is Indonesian, his acknowledgement of you must have been confirmed by a court before you turned 18. If that is your situation, have the court decision ready.