As Black the opening begins with somebody else's move: first 1.e4 or 1.d4, and only then your choice. Collected here are the defences and countergambits for the second player - the Sicilian, the French, the Caro-Kann, the Slav, the King's Indian and others, each with a walkthrough on the board.
Black does not choose the first move, only the reply. Which means preparing for two different worlds at once: 1.e4 with its open files and quick piece play, and 1.d4 with its closed centre and long manoeuvring. That is the main difference from preparing as White, where a single first move is enough.
The second difference is the goal. White fights to keep the initiative; Black fights to seize it or to equalise. Hence two kinds of repertoire - the counter-attacking one (Sicilian, King's Indian, Benko Gambit) and the solid one (Caro-Kann, Slav, Berlin Defence). Both work; it is a question of temperament.
In practice it helps far more to have a scheme you can play almost every game than ten variations for every occasion. A scheme produces recognisable positions, and recognisable positions come with ready-made plans instead of calculating from scratch on every move.
The minimum kit is one opening against 1.e4 and one against 1.d4. Here are workable pairs for different tastes.
Against 1.e4, the Caro-Kann: a sound structure, clear plans and few early tactical blows. Against 1.d4, the Slav for the same reasons. That pair covers nearly every game and requires no memorising of long lines.
Two at minimum: one against 1.e4 and one against 1.d4. That lasts a long time. A third is worth adding when a particular set-up from your opponents keeps putting you in an awkward position.
No. Black's task in the opening is to equalise, but equalising here means getting a game of your own, not defending passively. The Sicilian, the King's Indian and the Benko Gambit are all played for the initiative and for the win.
Develop by general principles: take the centre with pawns, bring out the minor pieces, castle. Against 1.b3, 1.f4 and openings like them, specific knowledge is barely needed - sensible play is enough.
A defence aims at a sound position and equality; a countergambit aims at immediate counterplay at the cost of a pawn. The Benko Gambit and the Albin Countergambit give up material for the initiative and for open files.
