Hi guys,

So I have finished building a simple guitar with one humbucker and one volume. When I play with the gain past half way and loud squeaking/squealing noise is generated. Any ideas?

I don't think it is the amp as when I plug in my other guitar there is no problem whatsoever??
There are a lot of things it could be.  It could be lack of shielding in the guitar, wires left too long, a bad solder joint, pickups adjusted too close to the strings or it could still be a problem with the amp and it's ability to cope with the level of signal you are sending to it.  Knowing what pickup and amp you used and seeing a pic of the g uitars wiring would help eliminate possibilities.  
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I'm pretty sure it's the lack of shielding. I've had the same issue when playing with my guitar close to a cranked marshall jcm800 attenuated to a quiet volume. But the magnetic fields from the transformers made it squeal anyways