I suggest two types of openings/defenses - mainstay and alternative.  Mainstay is your main opening (i.e. e4-Ruy Lopez or f4-Bird's Opening), your main defense to e4 (i.e. Sicilian or Caro-Kann), and your main defense to d4, c4, Nf3 (i.e. Slav or King's Indian).  Alternative are the openings and defenses that you experiment with and use to throw off your opponent that you play frequently against.  Having a good mixture of mainstay and alternative means that you are able to play either for a win or draw as needed.  For example, suppose your mainstay e4 defense is the Sicilian.  Wouldn't it be nice to have as an alternative defense the Caro-Kann for those occasions when you want to draw?  Vice versa, suppose your mainstay defense was the Caro-Kann but on occasion you needed to play for the win - have a pet line in the Sicilian!

     I suggest to new players they use the same moves in all games.  It is obvious why.

d3, g3, Bg2 for White

d6, g6, Bg7 for Black

     They can preface the White moves with 1.c4  Later on they can study the King's Indian, Pirc Defense and English Opening when they reach that phase where they wish to understand more.  They can also have a look at the Nimzo-Larsen Attack (where the games of Alexander Bagirov, Bent Larsen and Bobby Fischer will increase their knowledge) and Fischer's games with the King's Indian.  The c4 move blends in with the Reti Opening, Bird's Opening and the Nimzo-Larsen.  This is a system where they can activate the bishops.  Bagirov often employed a double fianchetto in the Nimzo-Larsen Attack.

   For advanced players, you might want to look at the games of Vladimir Bagirov and Alekander Shabalov (on Chessgames.com) playing the Slav Defense and the Semi-Slav Defense.  Very popular these days.  Bagirov was chess coach to Shirov and Shabalov.  He was also the Soviet coach of two lesser known players, Mikhail Tal and Gary Kasparov.     

I really like the O'Kelly variation of the sicillian, recently I play it a lot.

I post some common and interesting lines that I came up with with Game Explorer and memorized.

This opening is real fun, I'm bored with 3.Nc6!? :)

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One's opening should always suit their style of play. If you play an opening that lands you into an unfamiliar or uncomfortable middlegame, you will most likely struggle.

So, my advice is to go through your past games, assess your temperment and figure out your style of play. Then, go through games which have a similar style and check out the openings. Play them out in a few games and if you feel comfortable, continue practice. If not, repeat the process until you do. Soon enough, you will have a good repetoire. Also, learn the best way to counter these openings yourself. If you face an unfamiliar opening, opening principles are the only guide.

Practice is the only way one can improve so I suggest playing a lot of games. Soon enough, you will see an improvement(if you're sincere enough).

Wishing everyone good luck in their training,

Nightshadow