For converting an existing drupal5 theme to drupal6 theme you can follow these steps:-
=>Create an .info file

Create a themename.info file in your theme directory, which should look something like this:

The core variable should match to the value of DRUPAL_CORE_COMPATIBILITY constant which is defined in modules/system/system.module. Mainly you can use 5.x or 6.x

2) Enable the theme from themes admin path.

If you create the .info file it must be visible at admin/build/themes.

3) Replace block region variables

Make sure that none of your variables are named $left, $right, or $footer, and then make the following replacements:

1. $sidebar_left has become $left
2. $sidebar_right has become $right
3. $footer_message has become $footer, but when you output it, it should be echo $footer_message . $footer
4. $language has become $language->language
5. If you were using $base_path as a link to the front page, you can now use $front_page instead (which contains url(NULL))
6.
Drupal 5 Was

4) Add $signature variable to comment.tpl.php

Add an print $signature; after $content is printed.

5) Get rid of _phptemplate_callback() if you use it because it is no longer supported.

6) (Optional) Replace theme_my_hook_or_them_function() with my-hook-or-them-function.tpl.php

If your theme's template file defines any mytheme_somecode() or phptemplate_somecode() functions, you can optionally remove each of those functions and put its content in a separate .tpl.php file.

You need to rebuild the theme registry every time a .tpl.php file is created, removed, moved or renamed. you can do it by saving the thme page also. You do not need to rebuild the theme registry if the contents of a .tpl.php file changes. To rebuild/clear the theme registry (Drupal 6.0) simply visit or reload the modules page admin/build/modules

7) (Optional) you can Take advantage of new functionality

=> Customize your theme's admin/settings page
=> $body_classes can be more powerful than $layout
=> Themes can override Drupal's css files and css files from modules
=>Include right-to-left-specific css files (also see an example use of the new $language->dir in themes/garland/page.tpl.php)
=>use any updated jQuery 1.2.3 functionality
=> Submitted by user on date/time is themable
=> script.js in the theme directory is included automatically
=>Drupal now has a javascript function overriding mechanism