Step 1: What do you want to do? 

If you don't know what you want to do it's like asking someone directions to someplace without an address, or city, or even a state or the country it's in.

Step 2: What surface will you spray?

Though usually obvious knowing this is huge because it directly influences the next question you'll need to ask. 

Step 3: What paint will do that?

Paint is formulated for specific purposes.  Binder, glue, only adheres to certain surfaces, known as the substrate.  Another aspect to consider is the pigment.  Pigment is the dirt in the glue that gives paint it's color.  How finely the pigment is ground directly effects the amount of work necessary to keep your spray equipment clean and in working order.

Step 4: How big and how small of an area do you want to spray?

Spray equipment has upper and lower usable limits.  Those limits are elastic based on user skill.

In English.  The area sprayguns and airbrushes can spray generally falls within a range.  I like to call this the usable range.  You can make an airbrush spray a 4 foot diameter area, but you'll have to employ gravity's help and suspend the airbrush roughly 14 feet away from the surface facing down toward the surface being sprayed.  You'll also have to spray for a long time to get paint built up on the surface.

Step 5: Which airbrush or spragun sprays the kind of range you're looking for?

This is probably the hardest step of the whole method.  If you thought paint was a chore to go through and figure out what's what, this section will be even worse.  To comlpicate the problem more manufacturers have labrythine charts to 'guide' you to the right decision.  I'll give you the basic rules here though.  Remember, there's always exceptions to general rules, but this should give you a framework to recognize the exceptions instead of thinking the esxceptions are the rules.

• Spraygun = Large Spray Area
• Airbrush = Small Spray Area
• Gravity feed allows either moe viscous paint or lower air pressure, it also allows the gun to function on tiny amounts of liquid.
• Siphon feed allows quicker airbrush cleanup, quicker paint changes, and more paint before reflling the bottle.
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Tradeoffs:  Tradeoffs are a part of the equation.  If you maximize the ability in one area you lose something somewhere else.  If you maximize the ability to do fine work you'll lose the ability to spray large swaths of paint.