Themes

add theme
choose theme
configure theme
enable theme
find theme
install theme

Checklist when adding a theme

This is just a checklist for those who added a theme before. You find the complete explanation in the section: adding a theme. The links here point also to that section.

choose the theme
download and install
enable the theme
configure the theme

Checklist when choosing a theme

Use the Filter by compatibility: Make sure your theme is compatible with your Drupal-version.
Look at the previews: choose a theme that looks as close as possible as how your site should look like.
Examine the features: Is this the right theme for you? Does it provide the columns you want? AVOID themes that are build up with tables (look for table-less or CSS-driven). Tables were a good system for website-design, with CSS not anymore.
Look under support: are there any major problems with the theme?