Gavin,
Firstly I would strongly recommend you do NOT buy secondhand alloy wheels. You have no idea what use they may have had or even if they have ever been in an accident. For the price of new ones, ensure your life is safe.
As mewntioned you need the size of your exisitng wheels and this usuually stamped on the rim somewhere. Common sizes are 5J x 13, 5.5 J x 13, 5.5J x x14. You also need to measure the PCD diameter. This is the distance in millimeters between the centre point of opposing stud holes and common sizes are 112 mm. You alos need to know the offset and this is usually 30 mm.
If you plan on using the same tyre size then it is easy to get this from the existing tyres. I would recommend NOT using the existing tyres, even if they are fairly new, as again, it is just not worth it.
You can always take your existing wheels to a dealer and they will advise you the correct allow alternatives.