Assuming that you know about automobiles enough to know the different makes (brands) and models. So, let's begin with the specific car make that you have chosen. It's crucial not to get carried away by flashy models and variants that were launched with great fanfare by their makers but didn't impress the market. Buyer impulses can be irrational at times, especially when aesthetics become more important than the machining and performance metrics of the car. A car is after all a machine and its value lies in the quality of its engine, gears, suspensions more than anything else.
Howsoever good your knowledge of cars, you won't be able to figure out flaws in an engine unless you are a trained automobile engineer or an experienced mechanic or an automobile enthusiast who is obsessed with automobiles. In other words, you can't afford to overlook the technical aspects of a used car you're going to buy. Get hold of someone with any of the attributes mentioned above to accompany you for inspection of the car in order to be sufficiently sure that you're putting your money in the right car.
It's important to remember that there are car makes and models that you should avoid at all costs, especially since they offer very competitive prices compared to popular cars of the same category. For instance, sedans like Opel Astra, Opel Corsa, Peugeot 309, Fiat Siena, etc, if properly maintained, can offer prices that are too good to be true. Don't fall into the trap of making rationalizations like 'The car looks good, runs well and is unbelievably priced; so, why not?' About eight years ago, I came across a shining silver Opel Astra that was available for just Rs. 75,000. I nearly fell for it till I rationalized - if it's too good to be true, then it probably isn't good enough. Apparently, it had become a 'white elephant' for its owner.