Well, look no further than the first line for the first example of a metaphor in this excellent poem. Remember that a metaphor is a comparison, where one object is compared to something else, normally something that we wouldn't think of comparing it to. This is a direct comparison, in that no words such as "like" or "as" are used, as in the case of a simile.
Bearing this in mind, the first line compares the West Wind to Autumn:
O wild West Wind, thou breath of Autumn's being...
Note how this metaphor gives the West Wind an almost spiritual significance, as it is compared to the "breath" or that which sustains Autumn. Certainly the speaker is suggesting that the West Wind is synonymous with Autumn.
In the second stanza an extended metaphor is used that compares the effect of the winds on the clouds to hair:
The locks of the approaching storm.
This is an impressive image because it presents the clouds as "locks" or hair of "some fierce Maenad" being blown around by the West Wind, ushering in the storm.
Lastly, at the beginning of the third stanza, the Mediterranean Sea is compared to somebody waking up from a deep sleep:
Thou who didst waken from his summer dreams
The blue Mediterranean, where he lay,
Lulled by the coil of his crystalline streams...
Here the West Wind is pictured stirring up the huge waves as if rousing somebody from a long repose.
Here are three examples for you. Hopefully this will help you to find others in this excellent poem. Good luck!