You just need to reply to your thread and ask for it to be removed
It would be nice though to find out why you need your thread removed though (this can also be stated in the reply as well, on the thread)
Generally threads are not removed though, as they can help others with the exact (or similar) fault condition. So this is weighed up, depending upon the substance of the specific thread requiring removal.
Another point about PMs, as a "NewComer" still, you are not able to PM, this is to slow (or ideally stop) any spammers utilizing TechSpot just for PMing and nothing else. Therefore you need to be a regular poster here to gain PM status availability