Where did I first make a serious mistake in this game, and what was the correct idea instead?

You took with the wrong pawn on move 30 the correct move was exf5

i mean your first mistake was as early as move 2 but if we're not considering that then it's move 4 because you missed a free bishop

indeed

oh you're black not white

as well as moving your h-pawn

well then your first mistake was hanging the bishop on move 3

no he is black in this position

or she

Appreciate the corrections on Bg4 (move 3) and gxf5 (move 30).

First move was wrong, e4 was best XD