how to upgrade from 10.5.8 to OS X Mavericks?

You will need to purchase an OS X Snow Leopard DVD from Apple. This version is 10.6.3. After installing SL, you will need to run Software Update to update it to 10.6.8. You may not need to go any further if your iPhone is able to sync with 10.6.8.

No. You can purchase where ever it's sold.

Make sure your Macbook is upgradeable to OS X Snow Leopard before you purchase the DVD.

jadelim7154 wrote:

is there a diff in purchasing the OS from online apple store or DVD from apple shop?

The Snow Leopard installation DVD is only sold by Apple through the on-line store. The Apple retail stores have not carried the DVD for at least three years.

Buying from any other source can be risky as you may not get a copy that is unopened.

Any MacBook with an Intel processor can be upgraded to Snow Leopard...going higher you need to meet more stringent requirements. For Lion you must have at least an Intel Core 2 Duo, i3, i5, i7 or Xeon processore and 2 GB ram and 5 GB free hard drive space. Mountain Lion and Mavericks requirements are in http://www.apple.com/osx/specs

Here are the basic specs. Don't get confused with #5, 6. They are just features of Snow Leopard:

Snow Leopard General Requirements

1. Mac computer with an Intel processor

2. 1GB of memory

3. 5GB of available disk space

4. DVD drive for installation

5. Some features require a compatible Internet service provider;

fees may apply.

6. Some features require Apple’s iCloud services; fees and

terms apply.

jadelim7154 wrote:

How to ensure is upgradable?

Click the apple at the left side of the menu bar, then About This Mac. Under the Apple logo is the Mac OS X version installed, under that the Processor, and under that the Memory.

What does it show for the processor and for the memory?