Use the 50mm f/1.8 if you can get close enough. It’s much faster than your kit lens or 75-300mm, so it lets in far more light and is the best option for low-light stage shots.

For stills, shoot in Av mode and set the lens as wide as practical: f/1.8 for maximum light, or around f/2.8 if you want a bit more depth of field. Then raise ISO as needed to keep shutter speed high enough, because actor movement is usually a bigger problem than camera shake. A little noise is usually better than motion blur.

The 75-300mm gives more reach, but its smaller maximum aperture makes it much harder to use in dim light. The 50mm will work best if you’re near the front; if you’re farther back, you may need to accept either more noise or more blur with the telephoto.

A tripod won’t solve subject motion blur, so it’s not the main answer here. If you haven’t used Av mode much, practice beforehand so you’re comfortable adjusting aperture and ISO quickly.