Practice in the mirror. Try with basic emotions first, (happy, sad, angry, etc) and get used to how your face feels while you are making those faces. Also, study your angles so you naturally move your face to highlight your best features. Practicing these things will help posing come more naturally when you are getting your photo taken.
Also, if you have smaller eyes like I do, try to intentionally widen your eyes. Not enough to look surprised, but enough to make you look like you aren't glaring lol
Another tip is to keep your lips slightly parted. It keeps you from looking tense or bored.
You will also want to know how to find your light. Try not to tilt your head too far down; instead keep your head held high. You'll feel like you have your nose in the air, but it helps keep your face lit and prevents unfortunate double chin incidents xD
If you have a rounder face, try dropping your jaw slightly and lightly sucking in your cheeks. It will feel weird, but it will make your face look thinner. That is something you should practice too though, otherwise you might end up with fish face lol
Honesty my best advice is just to practice. Real expressions tend to be more subtle, but for photos you will want to really exaggerate your facial expressions. It's hard to do if you aren't looking at yourself, so a mirror really is your best friend until you get used to how your face feels in each pose.