Alright, to be honest, this is the hardest part. It's not necessarily required, but it may be helpful for some viewers. Feel free to skip it, though.
When preparing to build a man cave, the first thing you should think about is why. Why do you want to build it? Will you actually use it? What do you plan to do there? It would suck to build an awesome man cave, but never be able to hang out there.
The second thing you should think about is who. Who do you plan to invite into your man cave? It's probably not a good idea to invite your craziest friends, who will just trash the place. Trust me, I've learned from experience. Cleaning up after people is something you'll have to get used to, but to a certain extent, it's just plain obnoxious. Make sure everyone who is invited contributes somehow.
The third detail is where. You're obviously going to need a good place for it. Here's a good way to go about it: Take a look at your group of friends. Who doesn't have annoying siblings? Whose parents are the most lay-back? Who has at least some privacy in their house? Who has the most freedom? And of course, who has the BIGGEST room available? These are all good qualities in a host for your man cave.
The next thing you should know is what. What do you plan to add to your man cave? A mini-fridge? Lava lamps? A TV? Trophy heads? Find out before-hand some things that you think would be cool to add, but make sure everyone else thinks it's worth looking for.
Lastly, you need to know how. In order for this to work, everyone needs to pitch in. And trust me, ALL of your friends will have at least SOMETHING worth adding. Also tell people what to keep an eye out for, in case they see a good deal on an accessory you want. Garbage picking is also an option, or you could ask for items or money for Christmas or birthdays.
NOTE: Make sure you get a parents' permission. Not all parents are okay with leaving the renovating to their teenage son and his friends.