to be honest, google has been my main source for tutorials, youtube doesn't have the vast "wide web" that google can reach.
one google search can find at least 5 good sites on the first page, if your interested in making signatures then search "GIMP sig tutorials" if avatars are your thing, search "GIMP Avatar tutorials" and so on and so forth. also, expirement, once you do a tutorial, find ways to modify it, and make something that hasn't been seen before!
all in all, practice makes perfect, you're not going to learn all the features of the program in one night, and you're not going to be an expert in graphics overnight.
a good thing about GIMP though, is that it's similar to Photoshop, while the paths to certain features may be different, through some searching in the program, I'm sure you can find the GIMP version of a Photoshop filter, morph, etc.
good luck!
~stups~