JR Seishun 18 kippu

Could you give me information about terms:
March 1 to April 10 Valid
February 20 to March 31 On Sale
so, what is way to get it in situation,
that I will come to Japan after 31 March
but I will use it before 10 April?

You can go to ticket shops and buy seishun 18 tickets, including partially used seishun 18 tickets. There are ticket shops in many places e.g. the building at the Karasu.... guchi at Shimbashi station, in Ameyoko, at the West exit of Shinjuku near Uniqlo.

As your plan your trip, please make sure, though, that Seishun 18 Kippu really makes sense. They make sense only if you don't mind spending hours on local/rapid trains, have plenty of time to spare, but want to go on budget. Please note that at times night buses/long-distance buses provide comparable low budget travel too.

Note that the ticket shops are non-official ticket resellers. The tickets will be good, but are technically second-hand.

Which air port arr?
Where to go and from which stn by 18 kippu?

18 kipppu won't be sold out purchase at JR ticket office/window
or reserved ticket machine until sales hours of 31 Mar.
So find un-official ticket(kinken) shop as second hand after 1 APR.
But stocks not always this is very poplar sold out fast not so easy.