To attract monarchs to your yard, you should have both milkweed and nectar plants. Monarchs will only lay their eggs on milkweed and milkweed is the only plant monarch caterpillars will eat.
Check out my milkweed page to see what types I have in my yard and then hop over to my nectar page to see the various nectar plants I currently have. I seem to add more every year 😀