There's a lot of bullshit written about commas. A lot of hacks like Strunk and White have tried to make money by inventing rules that have no basis in what previous writers have done, or what makes sense, or how language actually works. (This is what a leading linguist, Geoffrey K. Pullum, someone who actually understands language, thinks of Strunk and White and other usage books: "untruths", "foolish assertions".) There are no rules. I can suggest a couple of principles, but feel free to ignore where it seems appropriate.
- Use a comma where you would insert a short pause while reading aloud (or while speaking generally).
This is a good rule if you're worried about parentheticals and adverbs and the like. Consider the doubtful "Actually, I'm not sure about that" vs the firm "Actually I'm sure about that".
Pauses are often used to divide up sentences or utterances, and commas can be used likewise. Even when joining sentences you may or may not insert a pause: "I went to the hotel, but I'm not sure why." "I went to the park but I didn't take the dog."
If it is a longer pause, or the end of a sentence, then probably don't use a comma. If it's the end of a thought, use a full stop (unless another punctuation mark seems called for). If you're not sure, you might use a semicolon, but lots of people hate semicolons so you could probably use a comma. A lot of this is style, or taste.
- Use a comma to avoid ambiguity or confusion. For this it helps to know the general principles of comma usage so you know what the reader expects. There are lots of lists of when commas "should" be used, and it helps to look at them so you know what is expected.
Example: in general usage, "Bring me, Jenny" means "Take me along with you" addressed to someone called Jenny, while "Bring me Jenny" means "Bring the person called Jenny to me". Use the expected comma here, to avoid confusing your reader.
On the other hand, feel free to violate convention if you want to surprise your reader.