You need to implement a foreign key constraint in your Task table.
Take a look at
this page[
^] from MSDN and also follow the links on the page. In the diagram The 'SalesOrderHeader' would be your Task table and the 'SalesPerson' would be the new table you should create for TaskType.
TaskType should contain at least TaskTypeID and TaskTypeDescription columns.
TaskTypeID should be the Primary Key NOT NULL. It should be an Integer datatype column and you can make it an Identity column if you wish.
Change the Type column in your Task table to be an Integer datatype.
Then follow the pattern shown in the link.