Before you create a new activity type, take a look at those that have already been defined for your team, as shown below, which are displayed when you select the Activity tab and click New:
Note: These are likely to cover most of your team's use cases, however, you may need to create new activity types and you may want to hide the existing activity types or finetune the fields they make available, as shown below.
You can customize the default activity types by going to Settings > Activity Types > New Activity Type. The existing activity types are listed here:
Click the activity type to see its definition. For example, the definition of the Task activity type is shown below.
With the activity type selected, you can do the following:
As you can see above, you can customize an existing activity type to meet your requirements.