For beginners, we suggest using an FTDI cable for programming and debugging, especially if you aren't simply porting an existing Arduino project to Pro Trinket. Much like many Arduino-compatibles, there's a header for connecting a 'classic' FTDI cable or FTDI friend.
The bootloader on Pro Trinket is dual USB & FTDI "optiboot" so you can use either the USB port or the FTDI breakout to upload. However, only the FTDI port allows you to use Serial debugging. So it's a bit of a tradeoff - USB is inexpensive, built-in but requires a button press to start & there's no debugging, FTDI requires an extra cable but has auto-reset and debug console.
Make sure you're selecting the FTDI-program type in the Boards menu!
You'll need to solder 6-pin header onto the FTDI breakout first. Match the black wire and green wire with the BLK and GRN markings on the PCB next to the FTDI header
Don't forget you'll need to install FTDI VCP drivers, available from FTDI chip. Then select the COM port from the Tools menu as well, to upload!
Of course, you can use the FTDI cable for debugging not just programming!
To use the FTDI cable to debug programs, add the standard
Serial.begin(baudrate); line as you would with an UNO and
Serial.println as you expect! Then select Tools / Serial monitor after loading your program. The Trinket will re-start when you launch the monitor so you can see your data right from the start; be patient while the Trinket finishes starting.