If you learn a few simple tips and tricks, getting goals and defending them will be much easier.
Understand positioning, defensive and offensive. When the opponent kicks the ball away, make sure that only one Brawler goes back to get the ball. Try to keep control of the area. You and a second brawler keep high ground. Understand the importance of passes, especially super passes to cover ground before opponents respawn and cover the goal. Learn how to stay in front of an opponent when defending the goal. Be careful not to get too close to enemies with the Ball, or they'll shoot through you to score.
Understand your teammates and learn ball awareness, that means you need to look out for which teammate needs help and when to move back. And where the ball is at all times. Learn your counters. Pick your opponents: If you're Shelly, go for Rico over Frank. Pick the enemy that is the easiest target and the biggest threat.
Learn to juke in general. Move unpredictably. Otherwise, your enemies will charge their supers.
Learn simple quick math. If a Poco is defending the goal, understand that maximum he does 924 per ammo, if he has full ammo he does 2772 at most. Lets say you're a Brock with 3000 health. Look at the numbers, understand how much damage each brawler can do, and judge whether you can walk through them and tank their damage.
Also, learn the weaknesses of your enemies: If Frank activates his super, shoot the Ball to the side because he cannot move while using his super.
Lastly: Don't just keep playing in hopes of getting better. Practice!