Turn off the gas to your hot water heater and allow the water in it to cool down before working on the vent.
Follow your vent pipe to the opening outside the house. Determine how long the vent pipe is and whether or not you can disconnect it and work on it separate from your hot water heater. If the pipe goes through walls, such as those in your basement, or is attached to your chimney, as some are, removing the pipe is probably not an option.
Check the pipe for obstructions by either peering in the open end, using a flashlight or by removing a section of the pipe. A clog could be made up of anything from accumulated soot to a small family of squirrels. Pipe can usually be removed by unscrewing one piece from the other.
Take the screen off the end of the vent pipe and clean it well with a sponge dipped in water and dish soap. If there is no screen, consider getting one to prevent rodents from entering your home.
Measure the diameter of the vent pipe and locate or purchase a bottle brush or chimney-style brush that can clean the inside.
Scrub the inside without water and then use a shop vacuum to clean out the debris and dust you loosened.