I will go through this quickly and hopefully get you started.
Spenser's sonnet 75 celebrates his beloved. He happily writes her name on the "strand," only to have the ocean wash it away. She says she will die one day, and her name will be similarly erased. The poet says no, his verse will immortalize her. The poem's theme is the ability of art to immortalize a person, so that memory of the person continues.
Shakespeare's sonnet 29 also celebrates his beloved. In his case, he is despondent at first as he compares himself to other men and feels they might have more than he does. Then he remembers his beloved and feels so happy that he would not trade places with kings. The theme of this poem is that love is the most valuable possession to have.
Spenser's poem focuses on his desire to immortalize his beloved, while Shakespeare's focuses on the joy and comfort his beloved brings him.
Both poems are sonnets and both follow an ABAB rhyme scheme ending on a final rhyming couplet, so their structures are very much alike.
Spenser uses a metaphor (a comparison not using like or as) to compare mortality to writing a name where it will be quickly washed away by the waves. He also uses alliteration, putting words beginning with the same consonants in close proximity, such as in "die" and "dust." Shakespeare uses simile (a comparison using the words like or as) to compare his joy at thinking about his beloved to a lark's song. He also use alliteration, such as in "like" and "lark" and "hymns" and "heaven's."