The most frequent reply to e4 is c5, if you intend to play e4 as your first move, you must know how to play against the Sicilian and our friends, the chess masters, have worked out a strong attack for us--the Grand Prix attack.
After 1. e4 c5 2. Nc3
After 2. ...Nc6 3. f4
After 3. ...g6 4. Nf3 Bg7 5. Bb5
A1a-- 5. ... Nd4 (not 5. ...e6 6. Bxc6 bxc6 7.e5 and 8. Ne4) 6. 0-0 (not 6.Nxd4)
A1a1--6. ... Nxb5 7. Nxb5
A1a1(i) 7. ...d5 8. exd5 a6 9. Nc3 Nf6
10. d4 Nxd5 11. dxc5 Nxc3 12. Qxd8+ Kxd8 13. bxc3 Bxc3
A1a1(ii)7. ...d6 8. d3 Nf6 9. Nc3 0-0 10. Qe1 Rb8 11. Qh4 b5 12. f5
A1a2--6. ...e6 7. d3 Ne7 8. Nxd4 cxd4 9. Ne2 0-0 10. Ba4 d5 11. e5 f6 12. exf6 Bxf6 13. Bb3
A1a3--6. ...a6 7Bc4
A1a3(i) 7. ...b5 8. d5 Rb8 9. d3 e6 10. Bb3 Ne7 11. Nxd4 cxd4 12 Ne2
A1a3(ii) 7. ...e6 8.d3 Ne7 9. Nxd4 cxd4 10. Ne2 d5 11. Bb3 dxe4 12. dxe4 0-0 13. e5 Nc6 14. c3 dxc3
A1b 5. ...d6 6. 0-0 Bd7 7. d3 Nf6 8.Qe1
A1b1--8. ...0-0 9. Bxc6 Bxc6 10. Qh4 b5 11. f5 b4 12. Ne2
A1b1(i) 12. ...gxf5 13. Bh6 fxe5 14. Ng5 e6 15. Bxg7 Kxg7 16. Rxf6 Kxf6 17 Rf1