download some brush packs, and some patterns, and put them in for you to work with. you can make your own patterns as well, if you take samples and turn it into what you want you can overlay it on a texture and make it really pop.
for example, what I like to do when I start a signature or something, is splosh some random colors out, grab an abstract type image, and overlay it and start working with the colors and brushes. it helps to have those at your disposal so you can make several layers and experiment with effects.
for example:
i made this a few yrs ago, but i used an abstract render to start out with and built upon it using brush packs, really pretty simple its just an eye for design and testing effects with your layers
using the patterns lol, and some brushes on top
an example of creating your own pattern XD
the best advice I can give is download some packs, and use LOTS of layers and just experiment like crazy till you get something cool. these are all from like 2008~ someone else can probably help on where to start and how to create your own textures n stuff