1) do I have to file and pay taxes if I make less than 11,000 per year?
2) if yes, which forms should I use?
Many thanks for your help and please let me know if you have good tax advisor in DC area especialized in G4 visa holders.

In my opinion, yes, you DO have pay INCOME taxes, but NOT self-employment taxes.

You would file as a non-resident alien, filing Form 1040NR and Schedule C.

If you want my professional help, double-click on my title above and scroll down to my profile/signature. You will find my email address and websites there.

Thanks a lot! And should I use the same form if I have my own business?

Yes

This is from IRS publication 519, page 54. Services performed by a spouse or minor child of nonimmigrant aliens with the classification of “F-2,” J-2,” “M-2,” and “Q-3” are covered under social security.
G-4 dependents get G-4 visa. They may or may not be covered under social security?

I think NO on the FICA (Social Security and Medicare) taxes.

The "G" visa is issued to those who work for international organizations, like the UN or IMF, and are usually exempt from ALL U.S. taxes, on the theory that they will pay taxes to their home countries.