While clicking the pictures, make sure your phone is stable. If the phone is not stable, then the pictures might come out blurry.
Keeping the focus on eyes will make the photos look very good and also it captures the details too which results in best portrait photos.
Lighting is an important factor while clicking pictures. Find good lighting as it makes the picture look good. Too dim or Too harsh lighting can make the picture look bad.
Smartphones come with In-built portrait mode which helps in blurring the background and focuses on you. This mode mimics the DSLR effect.
Background also plays a major role in Photography. If the picture has too many things in the background then it will distract the viewer. Simple background looks classy and only focuses on you.
Smartphones also offer manual exposure option. The exposure option will give a dramatic effect to the picture.
After clicking the pictures, you can edit your photos using photo editing apps. You can adjust Brightness, Exposure, Lighting, Contrast etc. through these apps.