APC or "All Purpose Cleaner" can be bought from any supermarket. Specialist APC car cleaners are available but a lot of people use Tesco's "Daisy", as its very cheap.
Can be used diluted 1 part APC 4 parts water for really stubborn dirt or up to 1 part APC to 10 parts water for lighter soiling or interior cleaning.
Depending on the strength APCs can strip any sealant or wax that has previously been applied to your car. If you are re waxing then a strong APC mix is a good idea as you want a clean base to start from.
Have fun, it can be addictive.
PS "Flash with fabreze" diluted 1 in 10 is a great interior cleaner that leaves the car smelling "cotton fresh"