What is a paragraph? Can I have one paragraph at least?

Of course you may have at least 1 paragraph. In fact, this reply will contain several paragraphs, of which this is one. The following paragraphs will discuss paragraphs themselves.

Paragraphs are used in formal writing to make text easier to read. After all, if you were reading a newspaper, magazine or novel, the text would soon become difficult to take in if the author presented his ideas and writing in one continuous block of words. So, dividing up the text into chunks of various lengths makes reading and absorbing information easier. 

A good paragraph will usually contain one central idea or theme and the next paragraph will develop that idea further, or present an entirely new idea. If you are reading a novel, a new paragraph may indicate a change of setting, the passage of time, or a different character or set of characters than feature in the previous paragraph.

Using Paragraphs in Your Writing

Paragraphs provide a structure for your writing which enables the reader to identify and follow the development of your ideas. Remember, paragraphs should have their own internal structure and fit into the larger structure of the piece. Be clear what the main idea for each paragraph is, deal with it fully, but beware digressions and irrelevancies. Good use of paragraphs is achieved when the reader can get a reasonable grasp of the content of your writing by reading the first sentence of each paragraph.

For more help on constructing paragraphs, watch this film: