Decide whether you are being asked to compare, to contrast, or both. If you are comparing, cite similarities. If you are contrasting, cite differences.
Get organized. Make a chart or construct a Venn diagram.
Give concrete examples from the items you are comparing or contrasting. Don't just offer a summary.
Use either a whole to whole approach or a point to point approach. Depending on what you are comparing or contrasting, after you've made your chart or Venn diagram decide which approach will fully answer the question.
Make sure to use terminology that clearly states the similarity or exactness when comparing. As an undergraduate student, I once stated that Greek Mythology was "like" a religion to the Greek people of the time. The instructor took off points because she said Greek Mythology "was" their religion. Be careful of your wording.
Show the differences when you need to contrast. Indicate exactly how the items are unalike or in opposition to each other. Do not get these mixed up with your comparisons.