How to choose and study chess openings

Costa's Web Page on Chess

Chess opening is certainly the most studied phase of the game but also the least understood by many chess players.  

Opening theory is so appealing and extensive that anyone is likely to be attracted by it; however it's easy to get lost among the several publications on the subject and to wrongly react studying long move sequences while losing the overall view on fundamental principles and basic ideas.

Some useful tips to start studying chess openings are given below as well as some more advanced advice on opening strategy.