Best way to make a sig is to just play around with filters. Hint: start with a Clouds filter, then just go into Filter Gallery and play around with stuff. You can also take a stroll through deviantART and download some custom brushes and use them to spice up your sig, then add some more filters. Add some brightness/contrast to your sig for a more intense look (another hint: use layer masks everywhere you can, makes life so much easier).
For characters, WoWModelViewer is your best friend. Make sure you choose a color for the background that doesn't appear in your character's armor; once you import the screenshot in Photoshop you can use the Magic Wand tool to cut out the background (use a feather of 1-2 pixels). Then it's all up to you to add some contrast/make colors brighter/whatever cool effect you wanna add.
For texts, dafont.com for a good font, then once you have your text make full use of Blending Options. Drop Shadow, Stroke, Bevel/Emboss, Outer Glow are some of the best effects. If you want the text to "blend in", set the text layer mode to Overlay and/or decrease the opacity. This trick goes for all layers; experiment with different layer modes and opacities.
And lastly, just googling "photoshop background tutorials" can bring great results if you don't feel like improvising
