Most Helpful Girl
Try a black tee-shirt or a leatger jacket. My mom has always told me that sideways stripes make a mid section look wider and she's right. Mixing sideways stripes with a poofy jacket isn't the best combo (that's why a sleek jacket will look better w/ that shirt) Use multiple colors too. Wearing all blue or red, etc. doesn't make your clothing stand out. Find colors that look good on you and that compliment each other1
Most Helpful Guy
Very casual and monochrome. If you uses more different colors it would look better. You can example wear black shoes, blue jeans, white t-shirt and black jacket for example, or gray t-shirt, blue jeans and brown or black shoes. Same colors is okay to look at if there's a contrast, so you don't need to go all rainbow.
Example on good color combinations: