Basically, you'll be COMPARING the two -- explaining how they are similar.
So start by brainstorming in two columns. Write down the details you see in the painting in one column; write down the details you read/hear in the poem in the other. You'll actually find yourself going back and forth as you think of things.
Once you've finished listing, identify the items in each column that are SIMILAR to the items in the other. You could use numbers, letters, symbols, or different colors of highlighters to help you SEE the correlating similarities.
Once you've identified the main (probably 3 or 4) similarities you want to include in your paper, you'll be ready to decide what order to put them in. And THEN you'll be ready to write a thesis statement.
Once you have the thesis statement written and the main points you've identified written into an outline or plan, you'll be ready to start writing your rough draft. Don't bother writing the introduction until after you've written the body of the paper. After writing the body of the paper and the introduction, you'll be ready to write the introduction.
That's the rough draft. Get ready to revise and edit before you write your second draft; you may end up writing more than two rough drafts!
Here are some websites that will help with various parts of what you need to do:
Sorry -- correction in this sentence:
After writing the body of the paper and the introduction, you'll be ready to write the conclusion.