Q: Mac OS X 10.5.8 (9L31a) upgrade

With the information that you have provided, I am not certain as to the EXACT model/year of your MBP.  If you enter the serial number here, the information will be provided:

https://checkcoverage.apple.com/

Any older MBP will at least run Snow Leopard.  You can order the disk from the Apple online store:

http://www.apple.com/shop/product/MC573Z/A/mac-os-x-106-snow-leopard

When you have installed Snow leopard, run the Snow Leopard combo update:

https://support.apple.com/kb/dl1399?locale=en_US

Depending upon the exact model of MBP, you should be able to install Lion or El Capitan.

Ciao.

Nnote that iCloud requires OS X Lion (10.7) or later.

The white plastic body late 2008 Macbook is limited to Lion OSX.  The aluminum body late 2008 will accept El Capitan.  Lion can be purchased from the Apple online store:

http://www.apple.com/shop/product/D6106Z/A/os-x-lion

Ciao.