You would be surprised what you can do with a single flash, even when mounted on camera.
If and when shooting girls say, then really soft lighting is generally more flattering to the skin tones.
If you were doing a portrait of maybe an old guy with wrinkly skin etc, then harsher lighting highlighting detail would be more appropriate.
Use light to create mood and atmosphere too. I'm not very fond of brollies actually. Try bouncing from the ceiling with a fill deflector on the flash.
Do your tests against a light background and make sure you keep notes. Then you will be able to see the different effects and know which
system created them. Its largely trial and error. Before long though, you will be able judge a situation quickly and easily.
Just have fun and don't pressure yourself.
PS... just remember angles.... Think of pool or snooker !