You could check out Bolter and Chainsword, under their decal project. They have tons of chapters already done, in the right sizes.
If you have an inkjet printer, you will need decal paper for inkjet printers, and a decal fixative spray to spray on the decals after their printed to 'seal' them to the paper, otherwise they bleed. Use white decal paper if your decal has white as part of it, otherwise, clear decal paper. If you're using a lazer printer, you can forego the fixative, since the fuser seals the decal, but you have to be sure to have decal paper for laser printers.
I've used microsoft word, powerpoint, and photoshop to size and print my own decals (flesh tearers). Word and powerpoint were easiest. I'd also suggest printing test pages on regular paper to compare to preprinted decals to make sure they're sized properly.
Micromark makes a decal starter set (
www.micromark.com), and I think Testors also does decal sheets, but I've never used them (the testors ones).