I also use MCSkin3D. Here is the link.

I also use Gimp which is also free.

As for tips, I would study how other people shade and incorporate it into your skins. Find a skin you like and use the shading as a reference (DON'T STEAL THOUGH)

sixfootblueI usually use MCskin3d (You can download it on PMC. Just search for it.) It's super useful, and has a lot of models that you can directly paint onto, including most mobs. You could also use it as a skin viewer while you paint your skin in an external image editor (Photoshop/GIMP/Paint.net).

+1 for MCSkin3D.

I usually use MCskin3d (You can download it on PMC. Just search for it.) It's super useful, and has a lot of models that you can directly paint onto, including most mobs. You could also use it as a skin viewer while you paint your skin in an external image editor (Photoshop/GIMP/Paint.net).

I'm not sure what to give in the way of starter tips, but for me it did help to watch people's work process either in tutorials or timelapse videos of their skinning on youtube. It also helps to read up on some pixel art tutorials if you just want to improve on things like colour selection or shading.