Do you want to go in depth or just something that will read and clear the codes for you.
If you want something simple just to read codes I would say get something like a scan gauge or an ultra gauge. The benefit of having one of theses are you can leave it plugged in all the time so you can monitor your car most importantly coolant temp as the gauges are just for show if you have one fitted.
If you want really cheep there are loads on ebay for around £15 mark very basic http://www.ebay.co.uk/itm/CAR-FAULT-...item3cd95e6b09
If you want something more in depth you can get a lead and dis software from ebay and the load it on to a laptop this will give you dealership level diagnostic, it's takes a little to get running correct but is very usfull
Hope this helps