What type of bulb do i need?

It is likely a sodium, Na, vapor or older mercury, Hg, vapor lamp. Inside the outer glass housing, there's a second "bulb" of quartz (clear), for Hg or alumina (translucent white), for Na. If you can read writing on the lamp or fixture with binoculars, you can try to buy a replacement lamp ("bulb"); most have Edison sockets and screw in like an ordinary household lamp. However, you'll need to identify the particular lamp type -- ratings vary from 70 watts to over 1,000 watts, and bases vary.

The lamp is not connected directly to the AC mains, but through a ballast, which serves a few purposes:

• It provides a high voltage (hundreds or thousands of volts) to start an initial glow discharge, which changes to an arc as the lamp heats and the volatile metal evaporates.
• It limits the current drawn by the lamp in the preheat phase and when operating.
• It times the preheating and controls the light, often with a separate photocell head.

If the fault is a dead lamp, it is easily replaced. A 70 watt sodium vapor lamp can be found for US$10. Use reasonable safety replacing the lamp, e.g., a lamp-replacement pole, and insulating gloves. However, replacing the ballast is not recommended, if you do not see an obvious way to shut all power at the breaker 9or fuse) box.