If car is dirty or covered in a light dust first thing id do is use a good snow foam as that will remove a lot of the dirt before washing it.
Id wash and dry it then and inspect the body work to see whats next plan of action.Look for swirls,tar or deep scratches.You can also get a very thin plastic shopping bag and put your hand into it and rub it over a clean part of the paint work and if it feels rough you would need to clay it.
If paint is in good nick and dont want go down the machine polishing road id use autoglym super resin polish first as is a good cleaner and filler for swirls and top it with a good wax like collonite if ya want a good durable wax.It is a hard wax to use if your not used to it but it.
Autosmart have a new product out now called topaz and is getting great reviews.Lot of people using it instead of wax now and lasting 3+ months and ya can do a car in 10 to 15 mins with it.Have used it myself and gives a great finish.
Here it is on a bonnet and few cars i done with Super resin polish,topaz and collonite.Bad iphone pics but ya get the idea