T-shirts have been around for so many years it's hard to remember when people started wearing them on a daily basis. The 1960s saw t-shirts becoming more personal and a lot more colorful!
Tie-dye kits were being sold for people to customize them however they wanted and in 1969 advertising genius Don Price produced and gave away hundreds of tie-dyed t-shirts to the performers at Woodstock. By 1920 the word t-shirt became an official American-English word and until the 1950s t-shirts were worn under rough clothing and for sportswear. You can check out this site https://rock-on.co.nz/collections/vintage-collectible for band shirts.
A Streetcar Named Desire brought them to the forefront of fashion when Marlon Brando wore one for the duration of the film in his iconic role of Stanley. It then became an outerwear garment and the plain white t-shirt is still a style staple to this day.
Due to its rise in popularity, business owners started advertising them and Disney was one of the first to print a character on a t-shirt.
The '80s was when they really started to become fashionable and Miami Vice almost made them formal wear! T-shirts would be customized by being torn, cut, and made into a vest. Women would wear them oversized over leggings and some girls even wore them as off-the-shoulder dresses.