How can I be US aircraft maintenance technician?
The FAA has a guide to becoming an aviation mechanic, and the related regulations are in 14 CFR 65 Subpart D. In summary:

Be at least 18 and be able to speak and write English
Have 18 months experience working on airframes or powerplants; or, 30 months experience working on both
Pass three tests: written, oral and practical

The experience requirements are in 65.77 and it doesn't say that you need to have worked in the US. So your experience in Pakistan should be valid, although I have no idea what documentation you need to provide.
If you're inside the US, there's no requirement to be a US citizen, although to work and/or study in the US you need an appropriate visa (or green card). Some employers might require citizenship for specific positions, for security or other reasons.
If you're outside the US, you have to be a citizen or permanent resident, or get special approval per 65.3:

§65.3   Certification of foreign airmen other than flight crewmembers.
A person who is neither a U.S. citizen nor a resident alien is issued
a certificate under subpart D of this part, outside the United States,
only when the Administrator finds that the certificate is needed for
the operation or continued airworthiness of a U.S.-registered civil
aircraft.