Writing from A to Z (fourth edition, p.496-499) provides a complete discussion of transitions. If your instructor has asked you to improve this area of your writing, your first response should be to read this section. One item you will find there is a list of "common transitional words and phrases." This is a very useful list; however, you should heed the following warning that arises because the list is classified by function rather than by part of speech. Do not attempt to use the words and phrases as if they are synonyms. Because the words and phrases represent different parts of speech they aren't always interchangeable. The way that each word or phrase is punctuated or positioned in a sentence depends on what part of speech it is. For example, nevertheless and although both show contrast. But nevertheless is a conjunctive adverb (AZ, p. 258) and although is a subordinating conjunction (AZ, p.480). Whereas as subordinating conjunctions begin dependent clauses that are followed by commas if they occur at the beginning of sentences, conjunctive adverbs are found all by themselves at the beginning of independent clauses, where they are most often followed by commas.

    Writing well is never easy; nevertheless, learning to write well is worthwhile.

    Although writing well is never easy, learning to write well is worthwhile.