One of the best ways is to work through a self-help manual. These are often pretty cheap in used book stores, and can be found easily online for cheap too. You will want to do your research, in case you find one that’s produced by someone that doesn’t have much expertise in the area. Another way to overcome social anxiety is to work with a therapist.
They’ll be incredibly helpful in working through your roadblocks. If you can’t afford either of these options, learning some deep breathing exercises off the internet may help you in a pinch. If all else fails, don’t immediately think of the worst possible outcome. This will take you time to train yourself to do; the old rubber band on the wrist trick may help you. You snap it when you think of a really bad outcome, and gradually come to a more rational conclusion.