Most instructions are in game.

Use the mouse to navigate menus, buy and build units, select weapons and aim and shoot at the enemy.  Remember this is a physics based game - your shots will arc so you need to aim carefully.

Upgrade your workshop and church to unlock new weapons and Acts of God.

Use Cursor Keys/WASD to scroll the screen.

Several keyboard shortcuts.  Eg. "R" for quick repair - tap "R" to repair the selected unit, if the selected unit doesn't need repairing, then the next most damaged unit will be selected - you can then press"R" again to repair that unit (so just keep tapping "R" to repair eveything).  "U" - upgrade, "B" - open build panel, "DEL" - sell unit + many more.  You will need to learn these to get the most out of the game.