do I have to buy a storage plan for each of my devices: laptop, desktop, phone? If I have bought a storage plan for my laptop, do i have to buy one for my desktop and Iphone too?

No. The extra storage is on the iCloud server, and the data there is accessed by all your devices, so the increasaed storage will be available to all of them.

Since you have come from the relevant Help page I assume you are an ex-MobileMe subscriber who has had the complimentary storage?

If you wish to continue your current storage level, the instructions are here:

http://support.apple.com/kb/HT5527

Scroll down to 'How do I purchase my current plan?' and expand the section applicable to your computer or device.

If you have not already done so you need to do it today.