Click to select a tower. If you have enough resources, a build button will appear; click it. When the tower is ready, you can place it on the map. The green circle shows its range. When an enemy is destroyed, it will leave behind resources. Mouse over them to collect them before they disappear. When a tower has taken down enough enemies (collected enough XP), it will glow blue. Then you can click it to select one of the 3 upgrade options. The 3 icons at the bottom-right of the control panel will light up, showing whether the tower selected can 1) hit multiple enemies, 2) slow enemies down, or 3) hit flying enemies. Take out all the toys before they reach the closet where you're hiding.