First of all, there are way too many variables. Whether it's your first time to Vegas, where you're staying, who's going, how much money you want to spend, and what level of partying you do all would factor in to any detailed suggestions. Let me assume since it's a bachelor party that you're staying on the strip. No relatives (dad, business partner, lame uncle) are going who would cause you to dial back the fun. Medium level of money, "medium" level of partying (you like to drink, but aren't going all The Hangover on this trip... We'll call it a starter kit.)
If it's your first time in Vegas:
Hard Rock, at least once. You'll probably go back. You can get all the partying/women/gambling/fill-in-the-blank there you want. Problem with it is, it's off the strip so it's away from the other big casinos. You may be tempted to wander across the street to Terrible's late at night or early in the morning -- DON'T DO IT.
Most casinos have a one-name bar that is fun on one certain night of the week. Ask a good looking bartender and she can tell you what night is where. Jet at the Mirage is pretty fun. Tryst at the Wynn has a weird crowd but usually very cool because they're all rich. I partied with Paris Hilton there. Pure at Caesar's Palace on Saturday night is a little touristy, but pretty fun and there are tons of women. And they are not shy. Every other casino has at least one fun night spot, so you won't get to every one, but it won't matter. Don't go to the Paris, unless you and your friends are gay. (Not that there's anything-- wait -- yes there is something wrong with that if you're going to Vegas for a bachelor party.) MGM, Bellagio, Mandalay, Wynn, Caesar's. Fun can be had anywhere. Even the piano bar at Harrah's is very good.
Definitely go to a rooftop bar at night. Ghost Bar and/or Moon at The Palms casino towers are okay. The Foundation Room (is that still the name?) at the top of Mandalay Bay is 'classy' (read: expensive) but awesome. Actually, my favorite last year was the Voodoo Lounge at the top of the Rio. It is more accessible to cheapskates and guidos but VERY cool since they've re-done it.
That should get you started, at least for Friday night.