Hi geniew,
Welcome to the Support Communities!
The link below will give you some pointers on how to customize the Mac.
Apple - Find Out How - Mac Basics
I believe what you are asking is how to change the size of a window so that you can see two applications side by side. For example, open the Safari application, and drag the bottom right corner of the window to resize it, then quit Safari. When you open the application again, the window will open in the smaller size you created.
Repeat for any other applications you wish, so that you can have windows open side by side.
Alternatively, you can switch between applications quickly as mentioned above in the link.
The Mac makes it easy to have multiple applications — Mail, Safari, iTunes, Pages, iChat, iPhoto, and others — open at the same time. So how do you quickly switch from Safari, let’s say, to Mail?
Just hold down the Command key and press the Tab key (Command-Tab). Mac OS X immediately displays a mini-Dock with icons for each of your open applications. At the left side of the mini-Dock, you’ll see the icon for your current application. Next to it (and highlighted), you’ll find the icon for the application you last used. Each time you press the Tab key (without releasing the Command key), you can cycle through your open applications.
One more tip: You can also use the Left Arrow and Right Arrow keys to navigate your open applications.
Or, use Expose to manage multiple windows:
Apple - Find Out How - Mac Basics
I hope this information helps ....
Have a great day!
- Judy