There are four types of transfer (decal) paper . You can buy them in packs of 12 A4 sheets. They cost about £12-£15 per pack.
Inkjet, white background
Inkjet, transparent background
Laser, white background
Laser, transparent background
The key thing to understand is that there is no white ink! The printing program assumes the background is white.
Any colour which uses white -- any grey for example, will be reproduced as a pattern of black dots over the background. Likewise a pink will use a pattern of magenta dots over the background.
The transparent paper will allow the base colour of the model to show through. Your lovely grey or pink will become a muddy or invisible layer over the underlying green or blue, etc.
If you use the white background paper you avoid this problem. However, you need to cut closely around the design, perhaps leaving a narrow white rim. This would be mitigated by placing your design on a grey faded background.
To give some examples;
This design was given a grey gradient outline, then printed on transparent paper. To put it on the model I had to paint a white block underneath it.
Final result…
You can use a variety of computer programs for making the designs. Photoshop (or equivalent) is obviously useful. You can also draw freehand on paper, then scan it in and overlay colour and so on.
The best advice is to practice and experiment to find out the best method for your designs. The paper sounds expensive, but it goes a long way if you are careful.
You will probably find Microsol decal liquid useful for applying large transfers to vehicles.