oooh good question. Well I believe the first opening people learn is called the "giuoco piano" meaning quiet game in italian. I would say people learn it and should practice it first because it demonstrates basic opening principles, like development, castling, and how to move your pawns. It can be a little boring, but nevertheless solid. An opening in the same vein is four knights.
A beginner should stick to 1.e4 at first, and then look into 1.d4. Another popular opening that can be learned and experimented with is the spanish (or Ruy Lopez) and the king's gambit. Both are enjoyable, and will provide the player with many different challenges. Another very versatile opening is the King's Indian Attack, which can be played aganist almost any defence black sets up.
As far as 1.d4, I would first learn about Queen's Gambit. That is a ridiculously popular opening, and worth learning about.
If you're playing black, learn the equivalent defences black sets up against the basic openings you're learning about (like giuoco and spanish) and practice with some others, like the French, or King's Indian (which is quite versatile).
To somehow bring this to an end, let me say that there are openings and defences which have volumes of modern theoretical material, and there are some you don't even see at the master level anymore. Ignore all this! If you are a beginner, it is important to learn how to play, not the latest theory. That has it's own time. Learn to walk before running right? Plus - you don't want to get swamped and overwhelmed with info. If you need any more info or clarification on some of these openings and defences I threw at you pm me.
Good luck and have fun!
What openings do you recommend for a beginner to study and your reasons why ? Is it because a particular opening is most frequently encountered ? Versatile ? Simple to understand ?
And how he/ she goes about building his/her opening repertoire ?