Who should I ask to write recommendation letters?

I want to start my green card process soon but I'm kinda worried about recommendation letters because I don't think I know a lot of ppl willing to write me the letters. Do I absolutely need recommendation letters for my case? If so, who are the best people to ask for recommendation letters?

to Wallace:

It's probably best to submit recommendation letters. I just asked well-known researchers in my field who know my work to write the letters but I guess it depends on your case

to Wallace:

I've seen some ppl said it will be helpful if you can get letters from US citizens. Do you know anyone in the US?