Writing essays requires a series of skills which include bringing together words and sentences int ocoherent and cohesive texts. The use of sentence connectors and text transitions (that is, words that help connect ideas in a text) is key to successful writing.
Here's a presentation by Anal Moral (anamoralj) I borrowed from Slideshare and embedded herein that you might find helpful:
Connectors from anamoralj
Besides, here's a thorough and complete list of sentence connectors where you can look up most of them - they're grouped according to their meaning and there's plenty of examples of connectors in use.
Finally, here's a large set of activities as they appear on eslflow.com so you can practise.