New H1B : Entering US criteria

[blackjack7]

Hi,

I have a new H1B received few days back and unfortunately it is valid only upto 13/04/2013. Is there any criteria of entering US 45 days before visa expires?
I am going to file extension as soon as I reach US as extension needs I94 copy in my opinion or can it be filed even if I have not gone to USA?

Thanks