While you may use most of the Activity Types within your database, it may be that you find yourself using one particular Activity Type more frequently than all of the others. In that case you might consider setting it as your default Activity Type in order to eliminate one click from your Activity creation process.
To set this up, go to your Preferences:
The second section you'll see on the Change Site Preferences page is Activity Management:
Click inside the Default Activity Type box and select the Activity Type you use most frequently:
After making your selection, click Save Preferences in the upper right hand corner of the Activity Management section and you will have successfully changed this setting.