Pin D13 has in internal LED & current limiting resistor, if an exernal LED is used an external current limit resistor should also be used.
Pins 0 & 1 are serial lines at TTL levels (no-activity level is High, Low indicates the start bit coming in); not true RS232, which can use levels from +/-3 to +/- 15 or more, the high voltage being the no-activity level.