Try to think about something that makes you happy when you get your picture taken. Don’t look directly at the camera - somewhere in the general area will suffice and keep you from feeling as overwhelmed. Don’t square up to the camera. Instead, turn your body on a slight angle and turn your head in the general direction of the camera. You don’t need to break out into a full-blown smile - try for a grin with your lips barely parted. Keep your elbows out away from your body, but not so much that you look like a chicken. If you have pockets in your pants, put about half your hand in the pocket facing the camera. Finally, take a deep breath and relax to avoid looking stiff.
Hope this helps!
You don’t state how much knowledge and experience you have, so I’m going to assume that you’re a beginner. You can take a picture in almost any lighting as long as you know how to set your camera for it.
Check out this relatively short tutorial on YouTube: Manual Camera Settings for Beginners
Another good resource is Tony & Chelsea Northrup’s channel: Tony & Chelsea Northrup YouTube
I hope this helps!