What body style do you need ( will you be carrying small or large loads ).
How many seats do you need ( how big is your family now or will it be in the next couple of years ).
How big an engine or powered by which fuel ( could be a major impact on the running costs ).
Do you prefer a manual or automatic gearbox.
Can you afford the insurance and other on-the-road costs. What is the absolute maximum that you can afford ( then allow yourself something in hand for extras you might really want ).
If on Finance or a bank or building society loan can you afford the monthly payments.
A useful formula is, in logical order, from the list of what you want, take out what you don't actually need, take out what you can't afford, and, if you're younger, what you can't get insurance on and what should be left is a much smaller, more manageable, list of those cars any 1 of which will suit your requirements - not your aspirations or fantasies - and be affordable to both buy and run.
This formula or one rather like it was expressed by that mathematical genius Albert Einstein where he calculated that E = MC2 or Envy equals a Motor Car double the size of your neighbour, workmate etc. Unfortunately this can be a very expensive formula to follow.
Once you've got down to a short list of 3 or 4 models it's worth doing a bit of background research before you go out and start looking. You will want to know what you are looking at, what it's performance is.. or was, what fuel consumption you can expect, what came as standard equipment and what it cost as new. You may also want to know whether there have been any problems with that model and what weaknesses it may have.
Most car magazines can sell you back numbers of road tests of your chosen car, usually for just a couple of quid, some organisations, including the AA, offer road tests by phone on premium rate phone lines for about the same cost and some others offer fax polling of road tests which, again, cost about £2.
The Consumers Assocation via their Which magazine comments from time to time on groups of used models and their problems and many motoring magazines publish such retrospectives fairly regularly.