parameters are passed in rcx, rdx, r8 and r9. Rest goes on stack after 32byte shadow space. This i understand.
But why do i have to align stack to 16 bytes?

Is it ok to do and rsp, 0xFFFFFFFFFFFFFFF0 at the beggining of program?

OS requires alignment and shadow space. If you can guarantee that state prior to API calls then you are in compliance. Leaf node that don't use the API can do whatever.

Ok, so basically i have to do anything to make rsp aligned to 16 bytes before any api call?

I think its better to do so at the beginning, and then keep track of rsp so its always aligned.

API will use the shadow space to store registers, so more that RSP aligned is needed! Look for my examples on this board - I have used some techniques you will like.

http://board.flatassembler.net/topic.php?t=10231
http://board.flatassembler.net/topic.php?p=94910#94910
http://board.flatassembler.net/topic.php?p=109871#109871

Good related discussion:
http://board.flatassembler.net/topic.php?t=11133