This is a very fair question- The answer is actually not so straight-foward. Openings and opening-theory is what is considered to be the "best sequence of moves" in a certain line or after any first white move. At lower levels you will notice that people will not follow this "best series of moves" the point is that there is a way to exploit this and gain the upper hand. The problem is that you have to have memorized exacly what to do on any deviation from what is considered "best" and that is the art of the opening. When you pick an opening to play you spend countless hours learning on how to exploit any innacurate moves by your opponent, of course this will not happen over-night. Simply playing with an opening will also give you a certain "feel" for the specific pawn structures that typically arise from that opening. You also learn the best plans to play in certain openings. The end result of playing and studying any single opening will give you a sort of "home field advantage" when you play in them. But you must spend alot of time and energy. Another thing to keep in mind is that since white has the first-move advantage the white pieces will more often than not be able to deviate from "best lines of play" and still have a decent game. With black on the other hand, you will both suffer the price of "opening ignorance" and reap the benifits of punishing even the slightest error.
How to practise openings?

The way to learn is by playing, and checking the opening that happened after the game -- you can check if you remembered to play what you decided to play the last time your opponent did this, where something you didn't know yet happened and what you should have done there, and whether you understand the reasons why.
By only checking after the game, you get several advantages:
- You automatically don't spend too much time on openings
- You don't spend time on openings your opponents never play
- Your brain has recently actually thought about the positions in question, making it more likely that you remember what you read.
It's possible to use free online sources (Wikipedia, chess.com Game Explorer, all sorts of sites for different openings), but if you'd like to buy a book, I recommend Van der Sterren's Fundamental Chess Openings over all others because it really does have only explanation instead of reams of moves, it will stay current for the next few decades, and it covers all the openings in one book.
Before you do too much opening study, you need to have a good grasp of opening principles. Control the center, piece development, king safety/castling, etc. With that foundation, it's easier to see why some opening moves are made and some aren't. When you see your opponent deviate from your opening plan, you can then put it into context, e.g. "He played h6 which prevents Ng5/Bg5, but this doesn't help his development at all, so I'll continue with another developing move such as Nc3. . ." Over time you'll learn how to deal with the junky ways people deviate from your chosen openings. You have to trust that in a majority of cases, there's some reason that a particular move isn't often played by stronger players.
Some of the ways of deviating, of course, are quite good and get played with some regularity. These deviations are separate openings or branches of an opening that usually have names, and you'll simply have to learn how to play against them if you want to play "your" opening. The deviations you'll need to look at are specific to the openings you play, but this is going to be an issue for just about any opening you play.
Play your opening choices consistently. Try to take a tiny lesson from each game (e.g. "I deviated here; the book move is X, and I believe that the book move is X because it does these things for my position.", "My opponent deviated here. The book move is Z. His move might not be as good as Z because his move has these pluses/minuses while the book move has these pluses/minuses. If I see this again, should I handle it the same way?").

Hi everyone,
thanks for your responses. I guess the simple and obvious answer to my question was indeed: There is no way to consistently make an opponent play "your" opening. (And I did really know that. LOL)
Your answers really helped me to look at the opening study somewhat differently and focus more on general principles and on what does and what doesn't (or shouldn't) work and apply those principles.
I have on occasion reviewed openings that happened during a game when I found them effective. Of course, I should do this *all* the time. It's a really good point to do so all the time as it will eventually help me focus on those openings that seem to happen more often than others at my level of play and rather than pick those openings that I like, I can focus on those that I simply MUST know more about during my games.
So in short: I definitely learned to look at this part of the game with a different mind set.
Great work, everyone.
Holger

OK, here's a confession: I never played Blitz. Ever. Always like to ponder my moves. Somehow playing chess quickly never comes across as chess to me. And the moves (and errors!) I make there would not be the kind of moves I make when I have more time to think.
But now I am hearing there is some training value with Blitz?
Hmmm... I never contemplated that before. Nice one! Thanks a million.
OK, here's a question that's been bugging me for some time. It may sound basic and it may have a wonderfully easy answer.... but I haven't found it yet.
I often read someone favours [name-of-random] opening, that somebody else is currently playing the [another-random] opening and most importantly that at the start one should only learn in detail one or two openings and defences and instead work on tactics or other areas.
Now the problem with me is that whatever opening I may want to play (or practise or focus on) in 9 out of 10 cases (or even worse) my opponent couldn't care less and cheekily plays something else.
How is possible to familiarise yourself in practice with an opening when chess is a game of two and the other person just keeps on interfering with my magnificent opening plan? How can anyone favour an opening.... if that opening barely ever shows up?
Scratching my head.....