Welcome to FlyerTalk, canoc_heou.

I am afraid upgrading on Cathay Pacific flights isn't that simple. You would first need to fly to earn the miles, and then the total accumulated miles you have earned over several flights can then be combined on an upgrade award.

You also cannot buy an upgrade. You can however buy a Business Class ticket.

You can join Asiamiles which is free, or any one of Cathay Pacific's airline partners who have a frequent flyer programme to earn miles. But Asiamiles is the only programme which you earn miles and redeem them for upgrades on Cathay Pacific and a few other airline partners. Information can be found on www.asiamiles.com

The Marco Polo programme is only worth it if you fly a lot and you want to earn elite level tiers. Information of the difference between MPC and Asiamiles can be found here: http://www.flyertalk.com/forum/11220622-post11.html