In the short term, rehearse well with as close a simulation of your eventual stage as you can manage, or use prescription or nonprescription anxiolytics, or try one of the many speakers' tricks for reducing stage fright. Most of the latter probably won't work, but some might.

In the long run, the best way is probably exposure: doing a lot of public speaking, perhaps in front of progressively larger audiences.

Consider this idea:

• How is public speaking different to giving a speech to an empty room, (or other similar but not stressful event). For bonus points actually do the non-stressful thing and afterwards consider what might be different.
• Make a list of as many of the parts that you can come up with
• Mitigate the parts that will be a problem for you in any way possible.

In this way a great beast of a "public speaking" problem can be managed to smaller think-able tackle-able tasks.