I can't think of any reason why you might play better or worse in an OTB (over-the-board with "real" pieces) game than on chess.net. Maybe you were just playing a really good player! The nice thing about a site like chess.net is you have opponents of all strengths available for you any time of day. Computer chess has made great strides on the user interface level....making it very easy to find opponents and actually play the game.

It's always good to get a clear advantage, so yes, if you can do so by making a sacrifice, then by all means do it. Just be sure there isnt a way to obtain the same or better advantage through more conventional means first. What i mean is, dont just sacrifice for the fun of it....if it's the only way to secure the advantage, then go for it. There are two major kinds of sacrifices: "sham" and "real".....a sham sacrifice is the more common, for example sacrificing a bishop when you know that two moves later you will win it back......a "real" sacrifice is the type i assume you are talking about, where you are sacrificing material for an advantage in one of the other elements of chess (space advantage, gain in time, saddling your opponent with weak squares or pawns, etc). These two types of sacrifices are explained in depth in Rudolph Spielmann's great work "The Art of Sacrifice in Chess".

I would start out learning the basic K and Q pawn openings, with an emphasis on tactical openings. The Ruy Lopez (1e4 e5 2 Nf3 Nc6 3 Bb5) is known to be a good opening for gaining an understanding of the role of the center in the opening. The Queen's Gambit would likewise lay a basic foundation of white's goals in the Q-Pawn openings, and prepare you for more advanced concepts later.

As black, I would recommend sticking with a few systems. The double K pawn openings (e4 e5) and perhaps the French defense, as the concepts are simple, and will teach you things that will be helpful to you as you progress. Against 1 d4 I would avoid complicated openings like the King's Indian for the time being. When you notice improvement in your rating, it will be a good sign to delve into more complex opening systems. The Slav Defense or Queen's Gambit Declined are good for the beginner who is answering 1 d4.