There is a kind of self-help club or fraternal organization called Toastmasters. It is devoted to helping people improve their public speaking. I see that there is a chapter in Jakarta. You should check out their website--even if you can't attend their meetings maybe they have self-help materials. Do a Google search for "jakarta toastmasters".

Just don't be afraid making errors or mistakes! You'll make them! Sure! What is the problem? How many English native speakers are only able speaking their own language? You are doing an effort! They will admire and appreciate you for this. If they don't they aren't clever enough for you therefore thanks to your mistakes you have spoken not only in public but also divided good people from bad people. Two times useful! I wonder how many errors I have just made! Ciao