You should usually begin your paragraph with a topic sentence.

The topic sentence tells the reader what you are going to write about in the rest of your paragraph. It makes a statement that needs to be developed with examples and details.  Be sure that what you write is a topic sentence, not just a topic.

        These are topics, not topic sentences:

                                            My friend Lo Van                 Lo Van
                                            Lo Van's activities                 Lo Van, my busiest friend.

It is better to state directly the topic of the paragraph than to tell what you mean to say.
 
                WEAK In this paper I will tell about my friend Lo Van.
                WEAK This paper will describe by friend Lo Van.
                WEAK In this paragraph I will show that Lo Van is the busiest person I know.
                BETTER My friend Lo Van is the busiest person I have ever met.
 

                            My friend Lo Van is the busiest person I have ever met.  Every morning he attends an English class
                at the community center. After class Lo Van works at a neighborhood market, and in the evening he is a waiter in
                his uncle's restaurant. Lo Van is even busy on Sunday mornings delivering newspapers. I am sure that at night his
                mind is hard at work dreaming of the next day's activities.

The topic sentence underlined above tells the reader that the rest of this paragraph will be about Lo Van and the reasons that make him the busiest person the writer has ever met.