"You started today."
Translation:Dechreuoch chi heddiw.
This is a statement, 'wnest ti ddechrau' is a question.
To use the auxilliary verb in a statement you have to either use the non-mutated form ie 'Gwnest ti ddechrau' (Gwest was a typo - thanks for spotting that)
or you have to start the sentence with 'Mi' in North Wales or 'Fe' in South Wales:-
MI wnest ti ddechrau