making my own

An ordinary colour inkjet printer will do. You can buy white or clear decal paper at the LHS or order online. There is a spray that goes on after you print to protect the ink. Can't print white though, you need an ALPS sublimation printer for that, which is discountinued. They can still be found on EvilBay, but go for a premium.

I don't think there is any software out there, you'll need something like Photoshop if you plan on doing any too intricate!

Depends on what kind of decal you want to do fancy non fancy like Bgrigg said you cant print white however you can get the testors decal making kit it comes with software and decal sealent and paper but i would buy new paper white and clear you can get it from your LHS. now when or if you buy the kit it will try and tell you have to buy a sepret disc to make your own tou dont have to just find the picture you want to make your decal out  right click select copy open the testors software pic any pic left click on the pic and press the delete button on your keboard now right click and paste it should past your picture you want to get the right size practice with normal paper until you get it the way you want it then put your decal paper in and print. If you want to custamize your picture use photoshop or any photo editing software i use print shop 8 bye jasc software you can usualy download a trial of there web site hope this helps, if you want to know how they look take a look at my post in Aircraft under ORION done i used testors decal maker for the tail.

http://www.beldecal.com/inkjet-instructions.html

The link has instruction on how to print decals on an inkjet printer, the site has instructions for laser printers.  They also sell decal paper for inkjets and lasers.

I use Corel Draw to produce my decals due to the fact that they can be resized with degrading the image.

You can use a free program called IFranView to prepare your graphics.

http://www.irfanview.com/