I decided to keep my standard head unit because it looks well in the car, is less likely to attract thieves than a shiny after-market unit and it is a factory upgraded unit anyway and i find it gives pretty good audio quality, it works with the steering wheel controls and has bluetooth and aux-in, so an after-market unit wouldn't offer a substantial upgrade.
Factory head units rarely have RCA or "Low line" outputs which your amplifier will need for it's input signal.
A "Low line converter" is a device which will piggy back your speaker outputs from your head unit and convert them to Low Line RCA outputs for your amplifier
The converter i bought also gives you a "remote wire" output which tells your Amp when the car is on so it doesn't drain your battery, if you get a converter without this option you can just join the remote wire to the Acc circuit of your car, or the 12V circuit with a switch (you will have to remember to turn it off though or your amp will drain your battery)
The methods i used will require an understanding of car/car stereo electrics so just be careful so as not to damage your car or yourself :)