How do you transcribe DNA into mRNA?
An enzyme called RNA polymerase reads the template DNA strand to produce an mRNA molecule.
A DNA molecule is double stranded. One strand of the molecule is the template strand and one is called the coding strand.
The bases will always pair A with T and C with G.
When the RNA polymerase transcribes the DNA it reads only the template strand. It will pair the appropriate nucleutides with their partners on the template strand of DNA using A,C,G and U (U replaces T in RNA)
Here is a video to help you better visualize how this happens. Skip forward to 5:23 to see the discussion of transcription.
Hope this helps!