I did this just a second ago, took 5 minutes to do, boredom got to me.
(https://rmrk.net/proxy.php?request=http%3A%2F%2Fbackup.manonas.net%2FSig.png&hash=7d992c76a891f64b3dc660f5a77ee41000f33abf)
Criticism is appreciated, since I may start doing this sort of thing.
Using three or four colours is usually a bad choice. It's very hard to make them blend properly. The text could use some work as well, and the signature itself is a little big. I like to use 400 x 150, but that's just me.
Try out some beginner tutorials, you'll get the hang of it. Pixel2Life and Tutoralized are good places to find tutorials, PlanetRenders has some good ones on their forum too.
Also, it has no border, be sure to add it next time. :)
I'm just gonna guess that the background is a stock image. Stock images aren't bad if you know how to use them. Try downloading some brushes from DeviantArt and brush in a background.
There's your criticism. :D