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This will be the first time I talk to a foreigner. I will work every afternoon around my area, so I can met some foreigner.

If they are walk alone. How can I ask their with a friendly questions ? ( Do you walk alone ? or Who do you go with ?...or what.. ?)

And then, If I want to walk with him, what I should say ?
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Hello, truonggiangnguyen—and welcome to English Forums.

Stopping a person on the street is not the best way to meet them. People who are walking are usually on a particular errand and often have time constraints. I suggest you try a bar or restaurant instead, where they are sitting down and perhaps have time to talk with you.

In any case, I suggest that you start with "Excuse me. I am learning English and I wonder if you have time to talk with me for a minute or two?"