Additionally, if you’re squatting, benching, cleaning, or jerking (perhaps pressing as well), don’t wear a tank top. Humans are sweaty and oily when they train, and you don’t want your sweaty, oily, and therefore slick shoulders in contact with the bar or the bench when you train. Friction is important, so don’t wear a tank top. Unless you’re performing curls - tank tops are then, of course, mandatory.
Pants
Sweatpants, shorts, leggings - all of these are acceptable. You do, of course, want to make sure they allow you to move freely and don’t bind up on you, so if you’re wearing shorts, don’t wear shorts so long that they come to or below your knees. Shorts of this length tend to bind up at the knees when squatting (and should be reserved for 14-year-old male basketball players anyway), so they’re a bad choice.