One of the most amazing aspects of the Internet is that you can easily reinvent yourself online. On the Internet, people primarily find out about you from what you tell them. Unlike in the "real" world, your first impression isn't usually how you look-- it's how you talk, or how your blog or MySpace looks. And that gives you a lot of freedom.
What kind of information should you put online? Well, before you add anything to the web-- a new screen name, a profile page-- heck, even registering for this website-- you should understand one thing about the Internet: once you put something up, it can be very hard to erase it.
Of course, most services like MySpace and Yahoo have "erase my account" features-- but, anyone else with a computer can make a copy of that picture/blog post/whatever you put up. And if they have a copy, they can put it up. Even something that you put up as a "secret" or "friend's only" on your website can be put up-- if one of your friends does it, or you accidentally hit the wrong setting by mistake. So, please be extra careful when posting personal information online.
A good general rule is never put anything online that you have to keep private.
Along those lines, many services (Yahoo, MySpace) ask you for a lot of personal information when you sign up. But you don't have to give it to them. As long as the information that you put in the form is valid, they will approve your account. So if a company wants your real name, or a zip code, just make up a name. And for your zip code, you can use any zipcode, like 90210 (yeah, Beverly Hills).
This is much safer than using your real personal information on everything, and the companies won't shut down your account because you put in incorrect information. So another good rule is, never give personal information to companies that you don't absolutely have to (instead, just make it up).
An exception to this rule is the year you were born. While you can fake the month and day of your birthday, many companies use the year of your birth to allow or block access to certain services, and they are often required to do this by law. For this reason, it's best to go along and put in your correct birth year when signing up.
Whether you use AIM or Yahoo or Skype or one of the many other IM services, or MySpace or FaceBook or other social networking sites, there are a lot of opportunities to meet new people online. A lot of the people you meet can be very interesting, and most of those that aren't you know right away that something is wrong.
The most important thing to keep in mind is that on whichever service you use, generally the information you put in your profile is publicly available. That means you should never put anything like your real name, address, phone number, etc. in your profile. Also, you might think twice before putting up pictures-- just make sure it's a picture you wouldn't mind everyone being able to access (and have a copy of).
When you're chatting with someone for the first time, giving out your a/s/l (age, sex, location) isn't really a problem as long as you remember two things: 1) you never really know if the other person is telling the truth and 2) when you give your location, just give the state, or the part of the country (midwest, west coast)-- not your hometown. If you want to exchange pictures, you can do that, too, but remember-- you don't really know who you're talking to. It might be better to talk with someone two or three times (or more) before exchanging pictures.
Once you get to know someone well, you may want to tell them more about yourself. This is normal, and fine, but keep a few things in mind: