A summary contains the most important details of what you are summarizing. Read the original and figure out what is most important. As for using six sentences exactly, you will just have to count sentences, and stop when you get to six.
The length of the summary depends on the length and complexity of the piece to be summarized. It is doubtful that a whole novel could be summarized in 6 sentences, but a page of a book or a short chapter might be summarized in six sentences.
A topic sentence is a sentence that opens a paragraph's. It is also always the first sentence in a paragraph. Topic sentences can be useful in a summary because if the topic sentence is not interesting a reader might not want to continue reading the summary.
Purposes of Topic Sentences To state the main point of a paragraph To give the reader a sense of direction (indicate what information will follow) To summarize the paragraph's main point answers: summary