How to smile at people (even strangers)?

I am sorry to hear about your present ignorance, immaturity and innocence. It all depends on various other factors. By the way what is your age now. I agree you have a valid and genuine doubt. In such situations, please follow your family tradition or the prevailing social customs.

Still, if you are really an innocent child, you can easily try to forget this episode.

JUST SMILE.

you need to relax first and be happy to give a natural sincere smile.
if you really like or love that person you will show the natural sincere smile.
"natural sincere smile" is an expression. It comes from your heart.
We all get a little bit nervous at times, but if you think that you really like that girl or think she is pretty or beautiful, that fear might fade away. I hope this helps :)
Remember nobody is completely confident when facing girls. you need to make your very first step to say to them and speak to them. As you commuciate with them, the fear will eventually fade away. and you might even try out funny jokes to talk about, to make everybody smile ;)
Hope this helps ;) I am sure the girls will understand "the nervous state" and stuff like that. It is just normal, get over it. you need to have a little bit more of confidence, don't worry, I am sure you'll do fine and things will work out okay. :)

Imagine that girl is your sister, your mom, or somebody you love very much, then you can smile to her naturally.

understand that there is nothing to fear. If they are smiling at you, obviously they like you. So just smile back. Its not like they are going to make fun of you or something as soon as you walk by. But if u do smile weird, they might be offended. anyways, just think of them as friends and smile as you would to a buddy.

do not get any inferiority with you. your nervousness is due to the mental fact that what she would think in her mind..

Bring a friend along and ask her/him to stretch your mouth upwards.

Stop thinking about it, just do it.

It all depends on the situation. Say cheese to yourself next time a girl or anyone that smiles at you. Look at them in the eyes and be confident too.

Imagine all ppl as if they are objects.

You don't become a sociopath from this, don't worry.