I forgot to add an additional point. The triangle of 5e you might say.
Action Economy is King. The side with more Actions tends to win, so figure out how you can get more Actions (Summons) and deny enemy Actions (Disabling abilities/spells)
Party Synergy is Queen. Recognize what works well together. One party member using their Action to make every other party member hit a target *and* disabling that target is ideal.
Advantage Hunting is Princely. Learn all the different ways you can gain Advantage (high ground, attacking from the side or rear, Restraining/Enwebbing a target, certain spells like Faerie Fire, etc). Once you learn that, combat gets a lot easier. Make sure you are always attacking with Advantage, while finding ways to make your enemies attack with Disadvantage.
One more general tip. You can't take an Action if you're unconscious. Using an Action to help an Ally is trading one Action for another character's Action, generally a zero sum gain (Cure Wounds, Help action). However, trading a Bonus Action to get an ally up, well that's just tasty. Fancy way of saying Healing Word is the best spell in the game. Bonus Action ranged heal. A character with 1 hp deals the same damage as a character with 100 hp. Use Healing Word.
