Arranging your base is important. There are various strategies you can use, but a good starting one is to place your Town Hall and Silos in the center, surround them with weapons and traps, and surround that area with walls. The walls will stop enemies long enough for the weapons to deal significant damage.

Keep your team varied. While you won't unlock a lot of monsters right from the start, do what you can to learn the pros and cons of each class and build your team around a strategy, but continue to have variety. Filling your team with one strong class may work for a while, but stronger bases will have answers to your plan.

Don't feel pressured to respond to an enemy attack. Starting off, you're likely to be attacked by players who are much stronger than you. Retaliating and failing will hurt more than just letting them go.

Make full use of damage protection. After your base has been wiped out, you will be immune to attack for a set time. Use this time to improve your defense and build a squad. The moment you attack another human player, your invincibility will vanish and you'll be vulnerable again.