Need help with Validation Rule - If Subject begins with Email: then Task_Type automatically has Email appear.

Ah! Then you need a Workflow Rule / Field Update on the Task - 

1. Evaluation Criteria: created
2. Rule Criteria: formula evaluates to true
3. Formula: 

   BEGINS(UPPER(Subject), "EMAIL:")
   

4. Add an Immediate Action - Field Update
5. Field to Update: Task Type
6. Set it to - Email
7. Activate