I would avoid putting a web URL or Instagram account on a T-shirt. It almost never looks good, its usually intrusive to the design, and if the shirt design is good people just tend to ask “hey where is that shirt from.” Gotta let the design do the talking not... words on the shirt do the talking.
From the days I was making a big chunk of my income from shirts I remember the designs that were most popular were shirts and hoodies that were on neutral colors (black, white,medium grey, light grey, off-white). Having a bright colored T-shirt would be severely limiting your potential pool of buyers.
Going as simple as possible is also really helpful. A design that could be read from across the room is good.