There is a very useful website called Academic Phrasebank (https://www.phrasebank.manchester.ac.uk) from the University of Manchester. This gives examples of how to start sentences in an academic way for different parts of assignments.
It can be very helpful when you first start writing academic essays and assignments as you may not have come across academic writing before.
As you read more academic textbooks and journal articles, it should be easier to write in an academic style as you’ll be regularly reading work that is written in that way.