See PH p.66. Starting at second level a druid gains the Wild Shape ability. The rule says the druid may change into a shape "you have seen before", but this doesn't follow the druidic tradition in Nolaria and is modified below.
Wild Shape is limited to "beasts". This is interpreted to be a non-monster associated with the Druid's Circle (see PH p.68). At second level, Druids may not choose a beast that swims or flies. At forth level, a beast may not fly. At eight level, there are no such restrictions.
Druids who are not associated with a Circle may not use Wild Shape.
Wild Shapes must be learned. Through careful study or being taught. At second level, when the Wild Shape ability becomes available. it is assumed that the Druid has been coached and trained to assume two different Wild Shapes. Beasts may be chosen from those associated with the Druid's Circle.
The following wild forms may be chosen based on a Druid's home:
A player may petition for another shape to be added to a home territory, if that beast lives in that environment.
If the Druid has access to a mentor/coach/teacher, a new Wild Shape can be learned in one week of intense study. The teacher must be able to assume the desire shape in order to teach it. The teacher is usually of higher level, but that is not a requirement. A peer can serve as a teacher, if they have the desired shape.
A new shape can also be learned through observation. This is a slower process that involves watch a beast of the desired shape to learn its form, abilities, and habits. This process takes 1d6+3 weeks in an area that contains the desired beast.