The first, and most fundamental, thing is to pick something you like, that you are interested in. That way you will keep your motivation throughout the build process. If you pick, for example, a civil model, but your interest is in military aircraft, and you buy the model 'because it's the only one they had', or in your price range etc, then you will struggle to get motivation to start it, let alone finish it.
The second important thing is the skill level of the kit. This varies between manufacturers - some have a better reputation for parts fitment than others. Do your research on these before purchasing your first model airplane kit. The last thing you want is to spend your money on a kit that becomes too difficult to complete, or you lose your motivation due to bad parts fitment. Don't rush out and buy a kit that has all the bells and whistles as this will have an adverse affect, start with a basic model, with a low parts count.