Why is the start of my coding sequence ATG and not TAC?
This image from the Kahn Academy article 'Overview of transcription' might help:

Essentially, the sense/coding strand of the DNA encodes the sequence that is transcribed. The RNA polymerase binds to the antisense/template strand, for which the code is indeed TAC, but when it then transcribes this strand it is again complemented, giving us the AUG that is recognised by initiator tRNAMet (also known as tRNAfmet), as shown in this diagram from the Kahn academy article on 'tRNA and Ribosomes'.

(Note that this diagram is actually showing Met in the middle of a sequence, rather than the start. fMET binding looks slightly different.