In America, we call this "small talk". It's totally normal, but be careful because other countries don't do "small talks" with strangers. 

(In my opinion) it kind of depends if you're a woman/man initiating the conversation with a woman/man.

But anyway...

1. Find something in common with the other person...

           ---Are you waiting in line for the same order? Do you have class with him?...

2. Compliment them...

            ---Do you like their shoes, clothes, hairstyle?...

            ---Maybe their son/daughter/pet is very cute...

3. Talk about the weather...

            ---Is it so hot that everyone is eating ice-cream or fanning themselves?...

            ---Is it so cold that everyone is waiting for the bus inside the convenient store?...

            ---The weather's been changing and now it's finally the perfect weather...

Hope I helped a little bit. Good luck! :) 

Hi Nhi, it actually depends on the situation. For instance if you are traveling to another country alone. You really don't much have option but to start a conversation with a stranger to make friends. (based on my experience)

-You can start by giving a nice genuine smile and saying "hi" "hello" 

-Where are you from? (then the rest will come spontaneously )

Hi Nhi!

I agree with @Liya. Small talk often bring you  up to big talk. Maybe at first you can ask where they are come from. Then if you know something about their country, you can giving compliments or just talk about it.

You can ask about their hobby, what are they doing currently and so on. Everyone love to talk about themselves :). Find something common, laugh together. Just be an open minded and honest person. They will tremendously interested to be friend you and respect you :)