Ty decided he'd like to start by turning himself into a girl. He'd pretty much always wondered what it was like, with how differently girls acted and were treated, and ever since he'd started puberty and seen how different girls were becoming to him, that curiosity had grown stronger and stronger.
He smiled at the realization that now he could move freely between the sexes, and decided to lock his door so no one could walk in on him. Then he stripped off his clothes and stood in front of his bedroom mirror, naked. He looked over his male teen body and wished for himself to turn female.
While the last few wishes had been accompanied by vague sensations, this wish felt much more tangible. Ty felt a tingling sensation spread out through him, and before his eyes he could see things about himself start to change. His facial features softened, his cheeks and lips seemed puffier, his hair a bit longer, his neck narrower, his voice higher-pitched. His shoulders slunk inwards and his muscles and fat seemed to shrink until he seemed physically more petite. His thighs and butt seemed to fill out and widen in contrast to this shrinkage, and Ty noticed even his skin seemed to have smoothed, as it had less hair and was more supple.
Having reshaped most of Ty, the wish now set about inverting his sex characteristics. Ty bit his lip as the feelings intensified around his chest and crotch, and his nipples became sensitive. He stroked them curiously and they swelled more and more, with mammary flesh soon sprouting under them. Soon he had cute C-cup breasts. Further down, he felt an aching emptiness inside him, and felt his penis start to shrink away, growing more sensitive as it receded more. As his balls moved up inside to form ovaries, Ty realised she didn't feel like a boy anymore. She wished for reality to adopt to her being a girl just as the last changes affected her- the remaining nub of what had been a penis formed the bud of her clitoris and lips formed from his scrotum as it opened out into her vaginal mound.
Ty looked at her reflection and saw a girl who showed no trace of having been a boy just minutes earlier. She actually quite liked how she looked, all told- she could imagine boys would be attracted to her. (It was at that point she realized she was still heterosexual, but attracted to boys rather than girls now.) Ty noticed the clothes he had pulled off that lay on the floor had changed too- instead of a T-shirt, a pair of jeans and some boxers, they were now a sundress and panties. She put those on, and did a little twirl in the mirror- she liked how pretty she felt in these.