Hi Alessandra,

>have a D3100 , since I am a stay- home- mom -of -3 -year -old
>-twin -girls/ I have a minimum budget for photography which is
>my hobby-therapy.Anyway, My father brought me this awsome,
>prestine condition flash ( speedlite 300tl) which is part of
>his SLR Canon T80. I have heard @ my photography class that
>you should never but a canon flash on a Nikon camera or you
>might fry your Nikon DSLR. How true is that?

It depends on the Sync voltage of the flash unit.
According to Photo Strobe Triggering Voltages the 300TL is safe to mount on your D3100 or any other Nikon DSLR.
You will have to set the D3100 in Manual exposure mode and Manual ISO mode and the 300TL in Manual flash mode for it to work.
You will have to set the aperture on the camera based on the flash to subject distance using the table in the 300TL Manual. You could also use a flash meter or the trial and error method using the histogram.

I have a idea of
>using this flash as off camera flash or as slave one . I
>called B&H they recommended me to buy a
>Interfit INT492 Hot Shoe and Strobe Flash Trigger Set . I
>noticed that this Speedlite worths around $ 150 in prestine
>condition! Is that worth it to sell it and get me another one
>? Or Should I invest US$ and get me the interfit hot shoe and
>strobelight trigger? Thanks for your help!

To use it as a remote you will have to set your camera in Manual exposure mode and Manual ISO using the Speedlight in Manual flash mode too.
With a Nikon SB-600 or SB-700 you will be able to take advantage of i-TTL flash control and set the camera in any exposure mode you want to use.
If you have experience using Manual flash and Manual exposure you know what to expect. If not there is a significant learning curve, especially if you want to shoot quickly as in snap-shots. If you don't have experience, I would recommend selling it and putting the money towards a used SB-600 or new SB-700. You can then use any exposure mode and take full advantage of i-TTL flash control.
The SB-400 is only slightly more powerful than the built in flash and is not compatible with wireless CLS Speedlights.
Good Luck and Enjoy your Nikons!

Best Regards,
Marty