The paragraph is the basic unit of any essay. It should have a coherent theme, almost a small essay in itself: it should only be about one thing, and should clearly announce what it is about very near the beginning-either obviously: 'I will now talk about this ...' or 'having thus described x, I will now go on to deal with y' or more implicitly. 'The most important thing about [the topic at hand] is x.' 'x, on the other hand, also has a place in the scheme of things' [Essay then goes on to describe x]'. There should be nothing in it that is extraneous to the discussion of its set theme; everything should contribute to this, and anything else, however interesting or stimulating, should be avoided. On the whole avoid short paragraphs except as a rhetorical device to focus the reader's attention on a change of topic. A paragraph should be about half a typed page or so-on average.
When you've finished your essay, go over each paragraph and check it. Is it clear what it's about? Does the topic announce itself near the beginning? Are there any sentences or phrases that don't contribute to this theme? If there are, cut them out; either put them somewhere else where they do fit, or lose them entirely (this is where having a word-processor is a great advantage, since you can cut and paste material without causing chaos). Once you've got used to doing that, you'll find that you don't need to, since you'll be so sensitised to the need for lean and purposeful paras that you'll be writing them in the first place. Which is highly to be desired.
There's a good chapter on this in Kane, Thomas S, The Oxford Guide to Writing, (Oxford: Oxford University Press, 1983) which is in the short loan section of the library.