How do I get online for Call of Duty 4: Modern Warfare for PS3?

I want play online, how do I get online to play Call of Duty 4 to challenge online players? I want pay online, I am good even tough as rookie. I play my friend's online sn and I got 59 streak kills. I want my own so, can anybody tell me how? P.S. When I am online, it fun to get more streak kills, so far my record 59. Hope p that.