|A front or side fan should blow into the case. Side mounted fans though are usually not a good idea...they can disrupt airflow thru the case.|
For most towers, the psu fan is enough. The next step would be to mount a fan just under the psu to exhaust (blow out) warm air.
Air rises as it warms up; nothing you or I can do about that. So, the idea is ambient air goes in the lower front and warm air exhausts at the top rear. That airflow washes over the components inside the case, picks up the heat generated by those components, and spews it out into the room.