You need to upgrade to Snow Leopard first. Contact Apple to buy a Snow Leopard installation DVD.
Make sure your system meets Snow Leopard's requirements:
To install Snow Leopard for the first time, you must have a Mac with:
- An Intel processor
- An internal or external DVD drive, or DVD or CD Sharing
- At least 1 GB of RAM (additional RAM is recommended)
- A built-in display or a display connected to an Apple-supplied video card supported by your computer
- At least 5 GB of disk space available, or 7 GB of disk space if you install the developer tools
Once you have Snow Leopard, you can upgrade to whatever your MacBook will support.
There is no need for sequential upgrades. The reason Snow Leopard is a prerequisite for further upgrades is that it includes the App Store. All OS versions subsequent to Snow Leopard are distributed that way.