What circuit I need to lower 20MHz 3.3-0V square wave to 0.01-0V square wave for very low load driving?
You're only talking about 1 mW of power, so my first thought is that a pulse transformer of some sort would be the best approach. However, your application requires a very large turns ratio (330:1) that is not likely to be available commercially. Perhaps you could come up with a circuit that cascades 2 (18.2:1), 3 (6.91:1) or 4 (4.26:1) transformers.

For example, Pulse Electronics has a 4.25:1 transformer intended for ADSL applications that would get you very close to what you need (326:1), but you'd need 4 of them.