This is an interesting topic to write about, though I am not certain what type of essay you have to write (persuasive, reflective, informative, for example). More than anything, for any essay it is essential for you to get organized before you start writing.
One way to organize an essay with this title is to go through "a day in the life" of a medical lab technician. What does a day look like in this profession, broken down into times of day or whatever else seems sensible.
Another possibility is to categorizes the types of activities a medical lab tech might do (i.e., paperwork, patient work, clinical work, or whatever).
For a purely informative essay, you might write about things like the requirements and responsibilities of being a med tech; if you are writing a personal or reflective essay, then you kind of get to just tell a story.
In any case, the introduction must do two things. First, it should set the tone for the rest of the essay, capturing your readers' attention in a way that suits the rest of the work. Second, it must prepare the readers for the specific points you will be writing. Do this by writing a purpose sentence, something which is clear enough to follow for the rest of the essay.
One last thing to consider is who your audience will be:
Analyze your audience. Review your assignment sheet to check whether you’ve been assigned a specific audience to address in your response. If no audience is assigned, you can assume your audience is your teacher, a knowledgeable and experienced reader in the subject area.
Whatever approach you take, get organized and you will find the essay much easier to write as well as much more effective to read. I have included an excellent eNotes site (below) which is specific to an argument/persuasion essay, but I think it will offer you some helpful advice for any essay. Happy writing!