"Composed Upon Westminster Bridge" is a Petrarchan sonnet, as opposed to a Shakespearian sonnet or a Spenserian sonnet. Petrarch was a famous Italian Renaissance poet whose sonnets eventually became well known across Europe. Romantic poets appreciated Petrarchan sonnets in part because Italy was thought to be the hub of classical European civilization, and they loved the classics (i.e., all things relating to ancient Greece and Rome).
A Petrarchan sonnet has fourteen lines that are divided into two sections: one with eight lines and one with six. At the ninth line, the poem makes a "turn" (or volta in Italian) and begins to elaborate in a different way on the subject or, sometimes, introduce a new topic altogether. Wordsworth's sonnet has a more subtle turn. In the first eight lines he introduces the idea that he has never seen such beauty before and then describes the scene. In the last six lines he returns to the idea of unparalleled beauty, this time comparing London to the countryside. The rhyme scheme is fairly simple: ABBAABBA CDCDCD. Only one pair of rhyming lines is slant (not quite a real rhyme, but almost): "by" and "majesty" in lines 2 and 3.
The poem is written in a loose iambic pentameter, consisting of five ("penta") pairs of unstressed and stressed beats ("iambs"):
Dear God! | the ver|-y hous|-es seem | a-sleep.
But not all of the lines follow this pattern. The first two lines, for example, both begin on stressed beats: "Earth" and "Dull." This loose rhythm comes closer to capturing momentary experience and a conversational tone than a stricter meter would. Wordsworth tried to write how regular people speak, which is one of the reasons he is considered one of the first "modern" English poets.