If you see garbled text on the screen, try the following:
Check you're using the correct line parameters for your device. Confirm the baud rate, data bits, stop bits, and parity for your device.
Make sure the character encoding is set properly. Serial defaults to UTF-8 encoding which parses variable length byte sequences into characters. You may change Serial's character encoding within the preferences.
Reset emulation. It's possible the terminal has been switched into an alternate character set by stray characters received. Use the Reset Emulation command from the Terminal menu.
Change or disable emulation. The terminal may continue to get switched into the alternate character sets if certain characters are encountered in the data stream. If your device is not sending VT100 escape sequences you may prevent this by setting "Emulation" to "None" under settings.
If this still doesn't solve your problem, please let us know.