They laughed when I said I wanted to be a comedian...
They laughed when I said I wanted to be a comedian – they’re not laughing now.
This is a pun. The humor in this joke lies in the phrase: "they’re not laughing now" which usually is used to imply that the people were proven wrong, however, in this case, the phrase could be taken literally to indicate that the jokester is not a very good comedian.
The joke is self-denigrating in that respect, and there is also ambiguity - did the jokester mean it literally or figuratively?