I want to make a simple game so that I can play against my friends at school. The school uses macs. Also if you can't make online games with flash what can I use. Also can they be 3-d if I eventually get good at making games?
RuneScape uses a Java applet; this is a good option for multiplayer games.
However, AdventureQuest is a Flash game and I think it's semi-multiplayer.
There seem to be some decent results, both examples and tutorials, if you do a Google search for multiplayer flash game.
Check out the book ActionScript for Multiplayer Games and Virtual Worlds. I've only leafed through it, but it looks at the basics of setting up multiplayer games (and servers) using ActionScript (which is the language of Flash). You can set up a development environment and server for free (although one reviewer complains that the development environment doesn't work on the mac).
"Also if you can't make online games with flash what can I use? Also can they be 3-d?"
Well, you can certainly make games with Flash, and there are libraries such as Papervision3D that can help with the 3D part.
However, there is another engine that's been growing in popularity - Unity3D - that you should seriously consider. It's the same basic concept as Flash, a Browser plugin (although Unity can also make stand-alone executables), but is a lot more powerful, especially for 3D animation.