Aircraft signal and position decoder
   The first thing you need is a standard scanner that can scan between 130 mhz and 135 mhz.. the nation wide frequency is 131.55 there are plenty more local ones you can also check out. set the scanner to 131.55 and turn off the squelch. (never use the squelch when monitoring ). you will hear a very faint spitz sound that lasts for 1 second, that is the transmission. How often you hear it depends on how many planes fly by, if you are near a airport then constantly, many more during the day then at night . Now plug the external out of the scanner to the sound cards input, use the line in if possible, only use the microphone input if you get weak signals from scanner. Use of the interface circuit shown below is recommended but not absolutely necessary. If using no interface then use a .1uF capacitor as a dc isolation. Note: do not use the tape out of the scanner unless you use a small amplifier.  Now we will need the software program to decode and display the results .
 The one to the right  comes from England. the map can be replaced with one from your own location, full details at their web site. also a ton of information on monitoring aircraft. This site has four programs to choose from. They are all time limited demo's. However they are all of far greater quality. Use Airnav acars decoder 2 Here is link
There are a few other programs that can be found on the internet for decoding the planes signals but airnav is the best, better decoding and greater number of messages received. However Airnave does not have position plotting on a map. The one from England does, but it's decode rate is less then airnav and harder to adjust receive levels. So i recommend using airnave first, once you start receiving and decoding messages then try the one from England. there is also one that is brand new to the net. ACARSD the picture in the upper right is from it. It is free to all but if you get it and install it read the warning about win xp, like airnave you have to set file to win 98 compatibility or it runs so slow you think it is not working. so far i have no success using it, but i only tried once so i can not give a real opinion. It plots position on a map also. Use the link on the web page for the one from England to custom make a map for this one also. Here is link for ACARSD
If you have any questions or need help
Shown below is the optional scanner to pc interface. The use of a ferrite bead on the signal line is recommended but not mandatory, if used they should be placed as close to the pc and scanner as possible, preferably at the back of the jacks.  All parts are sold at Radio Shack