I guess you are going to use an LED? They all work on about 3V and will definitely need a dropping resistor. Some LEDs come in a mount with a resistor and you might be able to find one that matches your battery voltage - you really can't over-volt LEDs and expect them to last, so don't be tempted by something that's close.

Finding the resistor value is dead easy; just google 'LED resistor calculator' and pick one of the many choices. You will need a couple of details from the LED spec sheet and then the calculator will show you exactly what to buy.