This post tells you how to go about learning to drive an LGV. First of all you need to decide which size of vehicle you want to drive. If you passed your test before 1997, you can drive up to 7.5 tonnes on your normal car licence.
If you passed your test more recently than that and you want to drive this size of vehicle you have to do the C1 test. If you are paying for the test yourself and think you may want to drive larger lorries in the future, you may as well just go straight for the Category C licence as the difference in price is small and the test is the same, just in a bigger vehicle.
As the LGV licence is vocational and as a lorry driver you will be spending hours and hours on the road, it is important that your health is up to it. Before you can be given the licence, you have to pass a medical. This is not a very stringent test and anybody in general good health will normally pass it. Anybody with any of these conditions, must inform the DVLA. Once you have your licence, you have to retake the medical at age 45.
Basically, the rule for any large vehicle is that you have to be 21 unless you take the CPC. For more detailed information click here.