My question is how can a US citizen can apply for his parents to become american residents?

You must meet the requirements:

• You are at least 18 years of age.
• You are employed.
• You must provide tax return back 3 years with your income meet the requirements in poverty guidelines.. check in USCIS form I-864P.
  
  Make sure your calculation is correct, for example: If you have a wife and a kid, you want to apply for both your parents, your income must be sufficient to cover 5 persons.

To apply, go to USCIS Home Page , download form I-130 and follow the instructions in there.