Make sure that your microphone isn't muted by clicking/tapping on the microphone button in the bottom left corner of your screen.
If you have clicked on the button and you still cannot be heard, this is most likely due to a lack of headphones. Make sure that you've got wired headphones plugged in or bluetooth headphones connected. If you plug headphones in and people still cannot hear you, try exiting the application and re-entering it.
If you had headphones plugged in but people are still unable to hear you, please visit Audio Settings by tapping the hamburger bar (3 horizontal lines) and selecting "Audio Settings". Sometimes the problem is as simply as increasing your Output Volume on the slider, but other times you may need to enter "Advanced Settings".
Here, you can select "Enable Push to Talk". This will make it so you have to hold down the microphone button (or "T" if you are using desktop) when you want to talk.