Just to add something else...
I've had both petrol and diesel in the past (we actually have one of each at the moment).
If you imagine two identical cars, one with a petrol engine, and one with a diesel engine.
The diesel car will cost more to buy than the petrol car.
The decision on which to buy then rests on whether you will see the benefit of the cheaper motoring in the diesel car. If you do mainly town driving, or a small amount of annual mileage (under 6-8,000 miles), then the difference between running costs in the two will tend to narrow, potentially making the petrol a better buy in the long term.
Check insurance costs VERY carefully. The cost of insuring a really cheap car can in fact end up being more than the car cost in the first place! There's plenty of online insurance brokers that will give you an instant quote.
Modern diesels are very similar to drive to their petrol equivalents. Gone are the days of smoky diesels that are a pain to drive.
Of the cars I've owned or driven, the VW / Skoda / Seat / Audi diesel is a really good one to go for. The Skoda Fabia is an excellent small car (either petrol or diesel) and the general "skoda" stigma isn't so great nowadays (I currently have a Skoda Octavia!). Gone are the jokes on why a skoda has a heated rear window...
I guess you just need to have a seat in a few different cars to see which one's most comfortable!
Ben