You can either upgrade to Snow Leopard (which will get you the App Store after you run Software Update)... or you can purchase the USB thumb drive from Apple:
First, you must purchase Snow Leopard 10.6.6 or later. Then open the App Store application to purchase and download Lion. Second, be sure your computer meets minimum requirements:
Lion System Requirements
- Mac computer with an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor
- 2GB of memory
- OS X v10.6.6 or later (v10.6.8 recommended)
- 7GB of available space
- Some features require an Apple ID; terms apply.
Third, prepare for upgrading:
How to Install OS X Updates Successfully
A. Repair the Hard Drive and Permissions:
Boot from your current Tiger Installer disc. After the installer loads select your language and click on the Continue button. Then select Disk Utility from the Installer menu (Utilities menu for Tiger and later.) After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the installer. Now restart normally.
If DU reports errors it cannot fix, then you will need Disk Warrior (4.0 for Tiger) and/or TechTool Pro (4.5.2 for Tiger) to repair the drive. If you don't have either of them or if neither of them can fix the drive, then you will need to reformat the drive and reinstall OS X.
B. Make a Bootable Backup Using Restore Option of Disk Utility:
- Open Disk Utility from the Utilities folder.
- Select the destination volume from the left side list.
- Click on the Restore tab in the DU main window.
- Check the box labeled Erase destination.
- Select the destination volume from the left side list and drag it to the Destination entry field.
- Select the source volume from the left side list and drag it to the Source entry field.
- Double-check you got it right, then click on the Restore button.
Destination means the external backup drive. Source means the internal startup drive.
C. Important: Please read before installing:
- If you have a FireWire hard drive connected, disconnect it before installing the update unless you will boot from this drive and install the update on it. Reconnect it and turn it back on after installation is complete and you've restarted.
- You may experience unexpected results if you have installed third-party system software modifications, or if you have modified the operating system through other means. (This does not apply to normal application software installation.)
- The installation process should not be interrupted. If a power outage or other interruption occurs during installation, use the standalone installer (see below) from Apple Downloads to update. While the installation is in progress do not use the computer.
D. To upgrade:
Purchase the Snow Leopard Retail DVD.
Boot From The OS X Installer Disc:
- Insert OS X Installer Disc into the optical drive.
- Restart the computer.
- Immediately after the chime press and hold down the "C" key.
- Release the key when the spinning gear below the dark gray Apple logo appears.
- Wait for installer to finish loading.
E. If updating:
- Download and install update(s)
- Use Software Update, or
- Download standalone updater(s).