Well, the USB port of the Arduino board looks just like a serial port to the OS, so if you're just hooking Arduino up to a computer, I'd recommend using its built-in functionality.
If you want to hook the Arduino up to something else that speaks RS-232 (the standard serial interace you're thinking of), then you do need a "level shifter" or "RS-232 transceiver" (pick your terminology). The most common way to do this is with a MAX232 chip. If you can breadboard circuits, you can whip up one using the MAX232 pretty quickly.
If you want something more robust, check out one of the various serial adapter boards like http://www.pololu.com/products/pololu/0126/
Note, if using a serial adapter board, you can't program the Arduino with anything hooked up to the RX/TX lines. Be sure to disconnect your serial adapter before programming.