1. What sort of bike is it? You'll probably need a crank puller, and a bottom bracket tool. Its not the easiest job if you haven't done it before. What is wrong with the crank that makes you think it needs replacement?
2. WD40 contains a lubricant. Your disc brakes work due to friction. If you lubricate them, they can't work. I hope you aren't working on cars too... You will need to clean them thoroughly, and hope you haven't destroyed the brake pads by impregnating them with lubricating oil. I recommend getting a can of Brake Cleaner at an auto parts store, and spraying the brake rotors and pads down, hopefully this will remove the WD40 and allow your brakes to function again. WD40 is not a good solution to most problems.
Replacement pads may be hard to come by. Promax is a department store brand, and I'm not sure where you'd get replacements. Your best bet would be to go to a bike shop to see if the pad is the same as a regular brand.
3. You aren't supposed to loosen the H and L screws... The position of those screws determine the Highest and Lowest positions of the derailleur. If they are both loose, then the derailleur will go too far to each side, and throw your chain off. You should shift all the way down, and then begin tightening the L screw until the cage of the derailleur is in line with the smallest chainring. Then shift all the way up to the highest gear, and then start tightening the H screw until the cage is inline with the largest chainring. You can adjust the cable tension by turning a barrel shaped adjuster on the derailleur, where the cable housing connects. But don't mess with that until you've set the limit screws properly.
4. Assuming your wheel or your fork isn't bent, then you just need to open the quick release, settle the wheel all the way into the dropouts, and then tighten it. You should have to use some force to close the quick release lever, not a ton, but enough that when you close the lever with your palm, it leaves a mark on your palm for a moment. If its too loose, tighten the nut on the other side of the quick release a little at a time until the quick release lever closes with enough force.
You should get a book on bicycle maintainence, and read this webpage:
http://parktool.com/repair_help/FAQindex.shtml
It seems like you are jumping into a few repairs without fully understanding them, which may cause you a lot more headache in the future. Its better to have the right tools, and to follow directions closely to make sure you don't miss a detail that could comprimise your safety.
peace,
sam