As well as spoiling the look of your car, dirty bodywork also reduces the value of your car. Chemicals in rain and corrosive winter salt mixed with extremes of temperatures really test the paintwork of your pride and joy. The grime has to be shifted if you don't want to have problems down the line when you want to sell your car. It's also good practice to wash your car as it gives you a chance to take a look at the bodywork and see if you have accumulated any chips or dings that need attention. You can also look at wheels, tyres, etc and make sure that they are in good order as well. So apart from treating your paint work to the wash that it needs, a bucket and sponge is a great opportunity for a quick check-up. In order to get off to a good start you need to make sure you have some basic kit. A hose (providing you aren't breaking any hose-pipe bans in the UK), a good bucket (with no grit inside it; give it a good wash out beforehand), a clean sponge, chamois leather and a squeegee blade (Turtle make one that Halford’s sell). When you are buying a chamois leather a good indicator of its quality is a strong smell of fish oil. Don't bother with the synthetic cloths - they just aren't as good - if they were, real chamois wouldn't exist anymore! The first step is to hose the car and shift the heavy mud or grime - if it's on there badly just let the water soak for 10 mins. Use a good car wash formula (your local Halfords will have some good deals - pick a good name like Turtle or Simoniz). Starts with the suds at the top of the car and work down, doing one side at a time (and rinsing with water in-between) this will prevent streaking and difficult to remove marks. Repeat this for the other side. Rinse again and squeegee off the excess water. Finally finish with the leather to get a squeaky clean finish to the body.
Online How to clean your car bodywork properly