A Black repertoire needs separate answers to White's main first moves. Begin with a defence against 1.e4 and another against 1.d4, then decide how you will meet 1.c4 and 1.Nf3. Look for structures you understand and positions you can practise regularly. Choosing a defence because it promises to avoid all theory usually leads to disappointment: every sound opening has decisions worth learning.
Against 1.e4, the Caro-Kann prepares a direct central challenge with ...c6 and ...d5. The French supports ...d5 with ...e6, producing distinctive pawn-chain battles. The Sicilian uses ...c5 to contest d4 from the side, while the Scandinavian challenges e4 immediately with ...d5. Compare their resulting positions before selecting your main reply.
Against 1.d4, the King's Indian permits a broad White centre and prepares counterplay. The Nimzo-Indian develops actively and pressures the c3 knight, but it requires White to allow 3.Nc3. You also need an answer to 3.Nf3, such as a Queen's Indian setup. A repertoire is a set of connected replies, not simply one attractive opening name.
Against flank openings, check whether you can reach a familiar structure without giving White a useful extra option. Do not force a favourite setup when the opponent's move order changes its meaning. Start with one main defence in each category, play it in real games, and add a branch when you encounter a gap.
Try this at the board
Compare the position after 1.e4 c6 2.d4 d5 with 1.e4 e6 2.d4 d5. Which bishop has a different future?