To activate the firewall in Mac OS X 10.4 (Tiger), follow these steps:

1. From the Apple menu, click System Preferences.
2. In the System Preferences window, click the Sharing icon in the third row. This icon is circled in white in the graphic below:
   
   
   

3. In the Sharing window, click the Firewall tab in the middle of the screen. The window should look similar to the one below:

4. If you see Firewall Off in the upper-left corner of the window, click the Start button below it to turn the firewall on. The window should now look similar to the one below:

5. Click the Advanced button in the lower-right corner of the window. The window will now look similar to the one below:
   

6. If a check mark is not already present, click in the checkmark box in front of Block UDP Traffic.
7. If a check mark is not already present, click in the checkmark box in front of Enable Firewall Logging.
8. If a check mark is not already present, click in the checkmark box in front of Enable Stealth Mode.

The window should now look similar to the one below:

9. Click OK.
10. Close the Sharing window.