When shooting in auto modes (P, A/Av, S/Tv etc), you will need to adjust exposure often. How? Quick rules of thumb for you today.
What to adjust. If the non-flashed part of your pic (what’s lit by available light) is too light or too dark, use exposure compensation (the plus/minus symbol). If the flashed part of your picture (what’s mainly lit by your flash) is too light or too dark, use flash exposure compensation (the plus/minus symbol with a lightning symbol next to it, or adjustable via the menu).
How to adjust. If what you see is too dark, use plus. If what you see is too light, use minus.
How to predict. When shooting a very dark subject or scene (coal mine), you will need minus. When shooting a very light subject or scene (snow scene), you will need plus.