There are two main things to consider when choosing an RC car. Firstly is the power source with electric, petrol and nitro available. petrol RC cars run on a mixture of normal petrololine and 2 stroke oil, both of which are available at virtually any petrol station making them very convenient and cheap to run. The petrol engines are big so petrol RC cars are only available in large sizes making them more expensive but this means there is also space for a large fuel tank so they have longer running times between refueling. The nitro RC cars run on nitro-methane which is available from all model shops. This makes them very fast without a high cost. The little nitro engines sound fantastic making the racing experience more realistic and thrilling. The nitro engines can sometimes need a little tuning and tinkering so nitro cars are usually the preferred choice of the hobbyist rather than someone looking for more of a toy. Finally there is electric. These are the easiest to use with no engine tuning required and the new brushless motor versions are as quick as a nitro car. The electric car batteries do not last as long as a tank of fuel does and then you must wait while you re-charge your battery but this can be easily solved by keeping a spare battery and fast charger.
The other thing to consider is the size. RC cars come in many different scales, from small 1:24 scale up to huge 1:5 scale. In general the bigger the model the more expensive it will be although a highly featured smaller car will cost more than a less featured larger one.
Whichever combination you choose you will find RC car racing fun and exciting and a fantastic hobby to get into.