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Youtube
Depending on the software you wish to learn about. Regardless, the ultimate website i use for tutorials and such is Lynda.com. They have a7 days trial for you to try, after that the fee is $25 per month. I highly recommend it, it is worth it.
Another, cheaper solution is to browse for tutorials on YouTube.
If you are unsure on what Video Editing software you would like to learn i recommend Adobe Premiere for Windows and/or Final Cut Pro for Mac.