Unsurprisingly, the errors here are all about commas. 🙂
The first line of dialogue contains a comma splice, a place where sentences are run together with commas but need end punctuation instead. The second line contains what we used to call a mild interjection, which ought to be separated from the rest of the sentence with a comma. (In this particular sentence, leaving out the comma means the speaker is telling the other person he/she does well at being no fun. I’m quite certain that is not what was intended.)
“I don’t want tomatoes. I want to go home!”
“Well, you’re no fun.”
I suppose you could also have an exclamation mark after tomatoes, but beware overusing such marks. When they appear too often or too closely together (such as having every single sentence end with an exclamation mark because the speaker is shouting), they quickly lose their effectiveness.