About debugging: by "serial port" we also mean the USB connection to the computer. So if you use the serial functions in Arduino (e.g. beginSerial(), printInteger(), etc.), that data will be sent over the USB connection to the computer, where you can see it, for example, with the Arduino serial monitor. Right now, the serial functions can only be used with the hardware UART (i.e. digital pins 0 and 1 on the board), but we're working on a software serial port library that would extend them to any pins.
For C syntax, just write the same C code you would in a non-Arduino ATmega8 program. E.g. PORTB, etc. should all work as normal.