Knowing well wear a shirt, an easy art to learn!
Everything will depend on the shirt if it is too long, it will be better to wear it tucked in. If you are small, carry out shirt may be more suitable and if you are too corpulent. If you want to convey a serious image, it is better to go in the pants but a pretty belt will be essential (ideally color matched to shoes worn). If by cons, you want a more casual look, wear the shirt outside the trousers is more appropriate.