Taking yourself too seriously can mean many things like you may have no sense of humor or you're too into yourself. In this episode, Dr. Miller, Tucker, and Nils explain how self-deprecating humor, making fun of yourself, is the best first step you can take to being less serious. Research shows many positives associated with this type of humor. 
Sometimes, being shy and reserved is a result of your social group. If that is true for you, find people or places where you are more comfortable to open up. Even having one great friend who is more playful to go out with is great. And as a last resort, ask people about themselves. Take a genuine interest in what they know that you don't know and build from there. 
They also mentioned a past podcast on communication that you can to listen in order to solve this problem.