Add resistors to the circuit to increase the total resistance. A higher resistance results in a lower amperage. The resistance of a resistor is measured in ohms. A resistor works by "resisting" the flow of current through the circuit. Be careful that you don't exceed the manufacturer-listed wattage rating of a resistor.
Lower the circuit's amperage by adding a variable resistance device or increasing the resistance on any you already have in the circuit. Variable resistance devices include transistors, FETs and rheostats, which are two-terminal variable resistors.
Reduce the voltage in your circuit to lower the amperage. For example, lower the voltage source from a 12V battery to a 9V battery.