How do I organise my games in Steam?

You can firstly move games you like playing often into your "Favorites". You do this by:

1. Navigating to the game in your library (note you can select multiple at a time)
2. Right click it
3. In the context menu that opens select "Add to Favorites"

If you're looking for more advanced organisation you can add your games into categories. You do this by:

1. Navigating to the game in your library (note you can select multiple at a time)
2. Right click it
3. In the context menu that opens select "Set Categories..."
4. In the new window that pops up, if the category doesn't exist, add it
5. Select the category to add the game to
6. Click "OK"

You can even hide games from being shown in the library, to do this:

1. Navigating to the game in your library (note you can select multiple at a time)
   2. Right click it
   3. In the context menu that opens select "Set Categories..."
   4. In the new window that pops up, check "Hide this game in my library"
   5. Click "OK"

Depressurizer is a third-party tool designed to make categorization easier, and it has some automatic categorization features as well.

I've only used it once, but I have no complaints.

(Just make sure Steam is not running when you use the tool, as described in their usage guide.)