By experience. By going through scenarios that make you anxious and fearful. For example, saying foolish, silly or inaccurate things, and being okay with the reaction.
Shifting your focus to your own response to things and things that you control helps. You can't control the reactions of others and honestly, I have great respect for people who say and do things and own their decisions completely. Others do too. Falling flat sometimes will also make you more likeable.
If you want to overcome this as soon as possible then you can take a flight club approach. "Pick a fight and lose." Do things you know will be goofy or embarassing(within reason), that will put you in the scenario or generate the reaction you're nervous about.
Doing this willfully and consciously will allow you to have full ownership of it, it will be in your control, which will reinforce the sense of okayness with yourself and create a sense of outcome independence.
What exactly is your anxiety about? Why is it that you think you have it?