Generally, you DO NOT need a comma after "therefore" if it is followed by an independent clause.
* Example: *Therefore, the experiment failed.*
However, you MAY use a comma after "therefore" if it is followed by a dependent clause.
* Example: *Therefore, since the experiment failed, we need to start over.*
Here's a simple rule of thumb:
* If "therefore" is connecting two independent clauses, use a semicolon (;) instead of a comma.
* If "therefore" is connecting an independent clause and a dependent clause, use a comma.
Let me know if you have any other grammar questions!