This is a really good question, and it demonstrates your interest in writing the most effective essay possible; so I commend you for asking. The short answer is that it depends on the type of essay you will be writing. The general rule for all essays--and any other type of writing--is to pick a tense and stay consistent. Shifting verb tenses is one of the most distracting things for a reader to endure; write in one tense and change tenses only to indicate a shift in time or some dramatic purpose.
Narrative essays are a bit of an exception to the rule because they tell a story, and the nature of storytelling is to shift sometimes between the past, present, and future. Again the key to this is to start in one tense and use it consistently until you have a reason to shift in time.
The Purdue Online Writing Lab is an excellent tool for all kinds of grammar and writing issues, and these are their exact recommendations:
- Rely on past tense to narrate events and to refer to an author or an author's ideas as historical entities (biographical information about a historical figure or narration of developments in an author's ideas over time).
- Use present tense to state facts, to refer to perpetual or habitual actions, and to discuss your own ideas or those expressed by an author in a particular work. Also use present tense to describe action in a literary work, movie, or other fictional narrative. Occasionally, for dramatic effect, you may wish to narrate an event in present tense as though it were happening now. If you do, use present tense consistently throughout the narrative, making shifts only where appropriate.
- Future action may be expressed in a variety of ways, including the use of will, shall, is going to, are about to, tomorrow and other adverbs of time, and a wide range of contextual cues.
For most descriptive, argumentative, and expository (informative) essays (which covers nearly every possibility), use present tense throughout, changing only when it is appropriate for effect or to enhance meaning. Happy writing!