I am running a circuit from a 220 20 amp gfi breaker in my panel out to a post for my pool pump. I plan on running two black 12 wires(solid) and one white 12 wire through sched 40 pvc 18" UNDERGROUND to a 220 switch (What wires get connected to the black screws, Line or Load?) and from the switch to my 220 outlet. The white wire gets connected to the ground screw of the outlet and into the gfi breaker at the panel, correct? As far as bonding goes if I use a metal outdoor box that is grounded do I still have to bond it? If so can I secure a bonding lug to the side of the box and connect it via #8 bare copper to the other bonding wires?

If possible you should run a green wire for the ground, white is for the neutral on 120 volt circuits. The wires from your panel will feed the line side of your switch and the lines to your pump will come off the load side. There are many kinds and types of double pole switches and if it's not marked, you may want to check it with a ohm meter just to be sure. Also made sure your wires are approved for damp locations.

Thhn is ok for wet locations, right? No problem with the green i can run a green wire to the ground screw of the outlet and into the nuetral hole of the gfi breaker, correct? How would you check the switch with an ohm meter?

THHN is approved for wet locations. With the switch turned off, you shouldn't get any resistance between the screws. When you turn the switch on you should get o.oo ohms between one top screw and the screw below it and the same on the other side of the switch. Do this with no wires attached to the switch.

THHN is not approved with wet locations. Only if it is dual rated THHN/THWN. Any insulation code letter with W denotes wet locations.

Tho most THHN is rated dual, there still is straight THHN out in the market.

Line terminals are always at the top of a switch.

tkrussell is correct. THHN by itself is only rated for damp locations. I always used the dual THHN/THWN.