Step 1:
Choose any Tower Defense game. The basic idea of the game is to build towers and upgrading them to destroy endless waves of enemies.
Step 2:
Place a tower close to the path of the game. A corner is better than a straight area while placing towers as the tower will be able to cover two sides.
Step 3:
Upgrade towers as you collect money and also purchase new powerful towers.
Step 4:
For destroying enemies with flying ability, purchase some towers with power to attach flying enemies when needed.
Step 5:
Try different strategy if you loose once. You will be a better player with experience.