Because academic writing is not designed to be innovative in style, it is possible to give a number of rules which you can usefully follow when writing. These are just practical consequences of the requirements to design for a reader and maintain coherence, and are really only rules of thumb, but if you find yourself deviating from them in any major way, you should ask yourself whether you have provided adequate motivation for this departure from the stylistic norm, or whether your writing is showing symptoms of an inappropriately writer-orientated perspective.
Impersonal forms of expression are preferred in academic writing. This does not mean that you should never use the word "I" in a university essay. The word "I" is quite acceptable in contexts where you are talking about what you are doing as a writer (eg in expressions such as "I am now going to discuss..."). The reason you may have been told not to use the word "I" (perhaps at school) is that you were being discouraged from taking a writer-orientated perspective in which the meaningfulness of assertions in the first person (eg "I think that fox-hunting is wrong.") is assumed is to be obvious. Making this kind of claim using the word "I", and without providing any evidence or supporting argument, is very like citing from unpublished sources, because it gives your reader no way to examine the basis of what you're saying. Using the word "I" in this way, in order to substitute for evidence rather than to clarify your approach as a writer is fundamentally in conflict with the reader-orientated perspective that is one of the expectations attached to essay-writing at university level.
Sentences in academic writing should not be more than about three or four clauses long. This is because the longer a sentence becomes, the more difficult it is to keep it in your head all at once, so that very long sentences will make it hard for your reader to understand you. In the eighteenth century, some writers, such as the historian Edward Gibbon, deliberately wrote long, or "periodic" sentences, because they wanted to make sure their writing was only accessible to an educated audience. This is not a good way to approach a student essay because, apart from anything else, this kind of long sentence is hard to write in a structured way, so your grammar is very likely to become incoherent.
For the same reasons, paragraphs should not be more than three or four sentences long. A paragraph should only contain one main point, and if the paragraph becomes too long, its overall purpose will become unclear to the reader.