I have recently gone into the texturing business.
However, when i make my trail transparant, it shows up with an ugly, REALLY UGLY White Background. On trails, this looks very shitty.
Any idea how to fix this?

Well, you have to save the image as a "PNG" file.

Make sure it actually is transparent and make sure you are using a file format that supports transparency. As said above, .PNG works well.

Maybe it's because the program you use doesn't support semi transparency?

All programs excluding MSP support semi transparency. Make sure the file format is .png, Make sure you are using an alpha layer below your actual layer. If this does not work then come back here.