A 2008 MacBook likely shipped with Mac OS X 10.5 something. iCloud is not available until Mac OS X 10.7.3, which requires 10.6.6 minimum to upgrade to.
That requires purchasing a 10.6 installer retail disc, updating to 10.6.6 combo or 10.6.8 combo to get 10.7 which is another purchase of $20.
Note applications written for 10.6.8 and earlier may not be compatible with 10.7 or later. Some 2008 MacBooks can upgrade to 10.8 for $20, or 10.9 or 10.10 for free from 10.6.8. They are the late 2008 Aluminum, Late 2008 Air, and all Pro models. Early and mid-2008 MacBooks are out of luck unless they are pro, and must upgrade to no more than 10.7.5.
Not knowing which MacBook you have makes it hard to tell you your choices. Use this tip to identify your Mac:
https://discussions.apple.com/docs/DOC-6413
In summary:
All 2008 Pro, Late 2008 Aluminum, Air 10.5 -> $20 10.6 -> 10.6.8 combo -> 10.8 for $20 or 10.9 for free or 10.10 for free.
White 2008 models, mid and early 2008 Air 10.5 -> $20 10.6 -> 10.6.6 or 10.6.8 combo -> 10.7 $20 -> 10.7.5 combo.
If iCloud is what is desired, or the latest Java.
If neither is important you can just get the 10.6.8 combo after the 10.6 purchase and be done with it. There are few other redeeming features for 10.7 and above.