Here are a few pointers to help you out.

It's important for a T-shirt to fit well across your shoulders.

If the seam drops off the shoulder and sits further down your arm the T-shirt is probably too big.

Equally if the seam is sitting closer to your neck then it's probably too small.

You want the seams of the sleeves to exactly align with where your shoulder ends.

The T-shirt shouldn't be too baggy or too tight.

If it's too big, it will drape and sag which can be unflattering.

Equally you don't want a T-shirt so tight that it borders on a crop top.

You want the tee to be cut so that it follows the shape and contours of your body.