When you enter combat, a list of your created and stored monsters will appear. Tap the type of monster you want to deploy and they should be placed. Monsters have particular talents, including destroying structures, bringing down walls, soak damage, etc. It is best to keep a variety of monsters on your list, rather than rely on one particular strong type. The battle ends when you retreat, lose all your monsters, destroy the entire base, or run out of time.
Defense
Use guns, walls, and traps and defend your base. The Sniper Tower and Rail Gun attacks enemy units when they are in range. Walls deter enemies into taking other routes or slow them as they are broken down. Traps are invisible until triggered, at which point they will damage enemies. Place your Town Hall and Silos in the center and surround them with weapons and traps. Then surround that entire area with walls. If your base is destroyed, you will become immune to further attacks for a short while. You can use this time to rebuild your defenses and put together another squad. If you attack another human player, your base can be attacked again. Allow the Mushrooms to grow and create natural walls that will help keep enemies away.
Monster buildings
Your Yard can hold three Monster buildings. The Monster Locker is where various classes of monsters are unlocked and upgraded. The Hatchery is where you will convert Goo into monsters. Housing is where your created monsters will be stored and deployed for battles. Goo generators will become very important structures. Have two high level Goo generators and one high level building of each resource gatherer. Position them in the middle of the map and surround them by other buildings to distract attackers. Those other buildings should consist of low level resource gatherers, the Town Hall and all the other buildings you can currently create. Place walls and defense towers around the protected buildings.
Resources
The fastest way to build your resources is to attack other players.
Town Hall
Your Town Hall's level limits what you can build and other unlockables. A lot of resources or Shiny is required to upgrade your Town Hall.
Upgrades
Build and upgrade the Twig Snapper as much as possible. Its resources are used frequently. Upgrade the Town Hall to level 2 as soon as possible. Unlock the Bolt as soon as possible. They will target resource building and is useful when looting other players. Use a combination of Pokeys and Bolts to get as many resources as possible.
Various Tips
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.
Battles...