Read the forums. Learn to use the search function. There’s a lot of wisdom here - and as you gain experience, you will be better able to apply the advice here to your own personal situation.
Pay attention to your play-style, and build your team accordingly. Choose your next hero based on what he/she/it will add to your current team.
Keep your expectations realistic. This game has a big luck element; sometimes you can beat a team much more powerful than yours, but not always. Sometimes you will get super lucky with summons, but definitely not most of the time.
And finally, remember - it’s only a game. It’s a fun game, some people take it very seriously… but in the end, it’s only a game.