Here's a breakdown:
* When "sometimes" is followed by a complete sentence: You do not need a comma.
* Example: Sometimes I go for walks in the park.
* When "sometimes" introduces a dependent clause: You do need a comma. A dependent clause cannot stand alone as a sentence.
* Example: Sometimes, when I'm feeling stressed, I take a long bath.
Let me know if you have a specific sentence you'd like help with!