The adapter you'll need is an RTL-SDR USB DVB-T adapter. Best ones seem to have the R820T rather than E4000. It should have the RTL2832U in it too. You probably don't want to spend more than £5 on this.
Depending on what USB dongle you get you may need an adapter of some sort to plug in other antennas.
Mine has an MCX socket (which is the tiny one). I already had one of these:
To make it a standard Belling-Lee RF plug. But you can get MCX-F-type if you have that sort of coax already made.
Some dongles come with a Belling-Lee RF socket instead already so you'll then need to decide what you want to break it out to.
My spider antenna has a BNC connector on the bottom so I have a BNC to Belling-Lee RF adapter. I also have various other adapters that go between BNC/F-type/RF etc in case I want to use other cables. Which I probably will because I have a flat t-type cable to use through the window.
My cantenna looks like this:
It is surprisingly good considering how cheap it is to make. A good first one to try if you have the bits. Simply a can, a female-female f-type adapter, some coax and some copper to pop in the top.
There's various threads here that have lots of info:
http://discussions.flightaware.com/ads-b-flight-tracking-f21/http://forum.flightradar24.com/forums/9-Technical-matters-(hardware)http://forum.flightradar24.com/forums/30-Flightradar24-(feeding-data-to-Flightradar24)EDITED: 3 Jun 2015 10:05 by ANT_THOMAS