Make eye contact with your listeners, but not for too long.
Making frequent but not overly intense eye contact when you speak will reassure your listener that you are interested in their response. In everyday conversation we naturally look at the people we talk to. It would seem strange if you were speaking to one person, while looking at somebody else.
It’s the same for all face-to-face situations when you use spoken language. You need to look at the people you are speaking to. You shouldn’t stare intently though as that is likely to make your listeners uncomfortable.
In one-to-one situations eye contact can be helpful to show that you are focused on a particular aspect of the conversation. It shows that you have something important to say at that moment and also that you care about what the other person is saying.
If you are presenting to a larger audience, you can move your gaze across your listeners as you talk, making only very brief eye contact with individuals, holding for maybe a second or so, before moving on. This will help your listeners to feel included.