If you want black decals, laser printing is your best option. You have to use special laser printable decal paper because the heat from the printer can release the film from ordinary paper and make a real mess. There are several sources of this decal stock, one is beldecal.com.
You can use inkjet for color. With modern water-resistant inks and being careful only to wet the decals from the back, you may not need a fixer. If you do use one, it is just an artist's fixative. I use spray cans of clear coat from Krylon, which come in gloss, matte, or satin.
For black decals you can use clear decal stock, but for color, you must use white decal stock and trim carefully around the decal, because color printer ink (inkjet or laser) is not opaque; it just tints whatever color the substrate is.
The exception is if you use a special type of printer called the ALPS which can lay down opaque white, colored, and metallic films. These are out of production, impossible to get serviced, and are hoarded jealously by modelers just like old Mattel vac-u-forms.
Another alternative is custom rub-on dry transfers, which are opaque. There are several vendors that you can send your artwork to and they will print them up for you. It is expensive. There is also at least one purveyor of kits to make them yourself,
http://www.pulsarprofx.com/DecalPRO/index.html. I have tried their product and it is fiddly but it works. Multiple colors with dry transfers is difficult, so what I use them for is to create a white backing marking exactly the size of my color design, which I then inkjet-print onto clear stock and apply over-top of the white transfer to make it opaque.
August