Which mantra to chant? How to chant?

[alum 0]

I'm interested in doing mantra jap. I'm a beginner in it so looking for advice from experienced persons like you all. I've many queries regarding mantra jap, which I request you to please answer:

 

Which mantra is most beneficial?

I understand that powerful mantras have to be kept secret, so how can I find a mantra guru to whom I can convince that I'll not misuse the mantras?

Has anyone here obtained a sidhi by repeating mantra 1.25 lakh times? Or anyone here has gained sidhi by direct kindness of a guru?

Can you share a mantra whose result can be objectively observed in a short time? -So to see for myself that the mantras actually work.

How to chant mantras for maximum benefit? I read a post here in which a person was confirming if chanting mantra onto a flower multiplies its benefit.

 

Please answer my queries or share anything for my benefit.