I tried it before and I didn't know what I was doing D:
Explain2meplz
First, find the "Reverser" and put it all the way to forward, or press and hold the W key. Then, find the "Train Brake" and release it completely (; key). After that, use the Throttle or Combined Power handle to make the train "LOLGO!". Remember, if you want to actually drive the train carefully, monitor your speed on the screepmeter and watch for signals on the track. Always keep one finger on the Train Brake key (; is less, ' is more.) The reverser can also control the locomotive's direction.
Other things: If you're travelling on snow or ice, remember to use the Sander in deisel/electric trains. That keeps the wheel's grip on the track, most of the time.
Bonus/fun stuff: Press and hold SPACE to use the horn/whistle, and press and hold B to use the bell. Automatically, the bell will turn on when the wistle is pressed. Release it with B.