What is the best defense against 1.e4?+
For most players, the Caro-Kann: it is solid, the plans are easy to understand, and it never goes out of fashion. The Sicilian scores better for players willing to study its theory, and the French suits strategic players. All three are fully sound.
What is the best defense against 1.d4?+
The Slav Defense is the most practical choice: sound, natural developing moves, and no locked-in bad bishop. The King's Indian is the best aggressive alternative, and the Queen's Gambit Declined the most classical one.
Is the Sicilian too hard for beginners?+
Mostly yes. Below about 1200, the Sicilian's open lines punish small inaccuracies severely, and its theory is the largest in chess. Beginners get better practical results from the Caro-Kann or Scandinavian, then can switch to the Sicilian once their tactics are reliable.
Do I need different openings for White and Black?+
Yes — as Black you respond to White's first move, so you need one answer to 1.e4 and one to 1.d4. Against rarer first moves like 1.c4 or 1.Nf3, your existing setups usually adapt, so do not study those separately until you actually face them often.