Well I used to be not very funny, then I started watching very funny people, how they do it. Everything, the choice of words, tone, topic, timing. First never tell a joke after a very funny one. No matter how good it is if you are not a champ at being funny it will be greeted with lukewarm polite smiles. The best to start with are your own "mishaps". In time I learned to tell every situation, no matter how dire as a joke in 20 minutes, sins it happened. It not only helped me go through some tough times but made a funny person out of me to. The best jokes are pointed at yourself and not some other person. When making fun of someone else it has to be with love and care. Knowing they will take it well. Today I am considered a very funny person. I was always positive and an optimist, this is just an added value.
PS: Never force it. Go with the flow. Good luck and practice!!!
