This is my first time that I'm studing a chess opening.
At first I thought I would have to know around 8 moves in a straight line, but after I started gathering information I was proven to be wrong. There are thousands of variations for every opening.

In this article I'll talk about the scotish gambit:

The main idea from any gambit is to get a better development than the oponent, at the price of some material.

At this point black have some possible moves like:
4. ... Bc5, with the idea to protect the pawn in d4
4. ... Nf6, giving up from the d4 pawn.

I'll try to focus on the first 8 moves.

[1] If Black plays 4. ... Bc,5 white have 2 main choices:

5. Ng5 - in this line white's attacks strongly, but if black defends well, they'll get a better game after the attack;

5. c3 - this line is recommended;

Against this line, black usually play one of the following moves:

5. ... Nf6 (transposes to a line of the Guioco Piano):

:

[2] 5. ... dxc3:

5. ... d3 (This move stops White building a pawn center with cxd4 and hinders the knight on b1):

5. ... Qe7?

More Variants [4]:

Else If black response to 4. Bc4 with 4. Nf6:

[3] Here are other variations for the 4. Nf6:

End if

Here's a match involving a scotch gambit:

In my point of view white will have to battle to develop his queenside

References:

To learn some bases about studying openings.
http://chess-training.blogspot.com/2006/11/how-to-study-opening-amateurs.html

[1] Great video with much information about the scotish gambit.
http://video.google.com/videoplay?docid=-5755617735639125927# -

[2] A Lazy PLayer's guide to the Scotch Gambit
http://chessteacher.110mb.com/2007/11/17/a-lazy-players-guide-to-the-scotch-gambit

[3] http://web.archive.org/web/20020803234939/www.pawnpusher.co.uk/scotgam2.html

[4] http://www.eudesign.com/chessops/sgam-app.htm

[5] http://davidmoodychess.livejournal.com/28114.html

Thanks to:
leziate and meniscuz for giving extra information about the opening