1) I am an accidental speaker :). When I started JSLovers it was impossible to get a speaker. My co-founder used to give a session and I used to manage everything. Then one day he was not able to give a session and I delivered my first session. It was the worst session. I didn't give any session for 3-6 months and then 1 day I just decided to give a session and since then no looking back.
2) I feel for me it was the zeal to just face the fear and also when you know something very well and not memorizing for faking you already know there is nothing to fear. Just to go and face the crowd.
3) My advice would be - "Do not go to public speaking because everyone is doing it. If you are passionate about knowledge sharing then only go. Also, there is nothing like a basic or advance topic there is always an audience for everything. All the best!!"