I am a fraternity alumni, and was in a fraternity throughout college. Here's some pointers based on my experiences. I should add that I was friends with the majority of my brothers before I decided to join. I suggest you get to know the fraternity you're thinking about pledging before you do it.
1.) As someone said, the financial aspect can be significant for some fraternities. For others, it's minimal. Regardless, if it's a chartered fraternity, you will have to pay new member fees (not cheap), and semesterly dues (not cheap either). Depending on your fraternity and if you live in their house, the ammount of the semesterly dues can vary significantly (some include housing and utilities).
2.) The rushing and pledging process for any fraternity is a significant time committment. While the rushing process is generally fun, the pledging process is not by any means. The difficulty of the pledging process varies between fraternities, and so does the nature of the process. But one thing is certain-- you will be hazed. The pledging process can last anywhere from 4-10 weeks (generally). Expect it to be easier for the first week, and to pick up from there. You will be expected to learn and memorize tons of fraternity information, do tons of ridiculous things, do tons of physical activity, and be at the beck and call of all the brothers 24/7. Expect lots of intimidation tactics, mind games, and tricks. The last few weeks will be brutal (hell week or hell weeks), generally culminating at the end with a single night of hell prior to your induction-- that night will be the worst of all. Expect to be dedicating virtually your life to those two weeks. Forget about making progress in school during that time-- it will interupt everything.
3.) You will not like all of your brothers, and not every one of them will treat you like one. The concept of "brotherhood" in a fraternity is not what you'd think it is-- i.e. it's generally not real. After about a semester in my fraternity, I realized that it's like one big disfunctional family. You'll have a lot of fake dudes that you dislike, have little in common with, and don't get along with. But you'll undoubtedly make a handful of very close friends. There will be bad apples in every group, and this is no exception. Only you can decide if it's worth having to deal with these assholes on a regular basis, and all the bulls*** that goes on in fraternities.
4.) ACADEMICS is the most important thing in college. Don't let anyone tell you differently. You are there to get your degree and to be successful, not to party. It is your responsibility to do your best in school. A fraternity will constantly be challenging your dedication to academics. This is a big disadvantage. Even if you try to divorce yourself from it as much as possible, you will constantly be getting sucked into fraternity obligations.