Select a shirt type that possesses plenty of fabric in the area you wish to cover. Be aware that certain movements will cause a shirt to bunch up or lay flat, so give yourself plenty of room for error. In other words, don't select a style of shirt that barely covers the tattooed area.
Consider the color of the shirt and its transparency. If you are only going to wear one layer, the top must be dark enough or thick enough to stop the image of your tattoo from showing through. Men may wish wear an undershirt to add another layer of coverage over upper body tattoos.
View yourself in the mirror making a variety of motions to determine whether your tattoo is solidly covered or requires more effort to hide.