1. Avoid using the flash as it can make the images look flat. Instead, use the existing light and watch for distracting shadows spoiling your shot.
2. Stay away from the zoom function; it makes images grainy and poor quality.
3. Don’t always rely on auto exposure. Usually a press and hold on subjects that look too dark is a big improvement and it will alter the focus of the shot to what you actually want to focus on.
4. Pay attention to composition by looking for patterns and symmetry, and embrace negative space. Keeping lines straight and symmetrical rather than at an angle will make your image look more professional.
5. Consider retouching the images in one of the many editing programs. The app Snapseed is a good (free!) place to start.