Applications:
Photoshop (Free 30Day Trial) :
https://www.adobe.com/
GIMP (Free) :
http://www.gimp.org/
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Renders / Stocks
Planet Renders :
http://www.planetrenders.net/renders/index.php
Anime Renders (Click Gallery) :
http://animerender.com/
DeviantArt (Only use renders/stocks that the author says is permitted for the public) :
www.deviantart.com
Google Image Search
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Brushes (for Applications like Photoshop and Gimp)
DeviantArt :
www.deviantart.com
BrushEezy :
http://www.brusheezy.com/
PSBrushes :
http://www.psbrushes.net/
How do you use these brushes?
For
Photoshop :
Go Into the Adobe / Photoshop Folder
Then go into Presets folder, and then Brushes
Put the Brush file (usualy .abr) into there, and restart Photoshop
For
Gimp :
Pretty much the same way as above
Go into the Brush folder in Gimp and throw the Brush file in there
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Fonts
DaFont :
http://www.dafont.com/
1001 Fonts :
http://www.1001freefonts.com/
How do you install these fonts?
Follow this :
http://www.dafont.com/faq.php#win
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Tutorials (Most Important)
DeviantArt (search Sig Tutorial) :
www.deviantart.com
SigTutorials :
http://www.sigtutorials.com/
Pixel2Life (search Sig Tutorial) :
http://www.pixel2life.com/
Google/Youtube/etc. Searches for Sig Tutorial
Here are a few Tuts I got Permission to Post, thus not violating any Ripping Rulez
Click
Click
Click
There are many other things one uses to make great sigs, example: C4D's, Filters, Pen Tool, etc.
Just play around and have fun :D
~Enjoy