Though this is a basic question (a few google searches will provide all answers) and you have asked a lot of questions, I shall answer them one-by-one. Why is RNA single stranded (and not double stranded like DNA)?. dsRNA is less stable than dsDNA. See this: http://en.wikipedia.org/wiki/RNA#Structure. read more
DNA is not always double stranded and RNA is not always single stranded. There are plenty of viruses out there that serve as counter examples to that assertion. Now the reason that DNA is generally double stranded and RNA is not is linked to their function. read more
Perhaps a clarification has to be made. both the strands can be used as template for DNA replication, but only the 3'-5' (anti-sense) is used during transcription. the reason being RNA polymerase only reads from the 3'-5' DNA strand and synthesize RNA in the 5'-3' direction. read more