Final answer:
The given sentence, 'First, make sure you preheat the oven,' is a simple sentence as it consists of one complete idea expressed in a single main clause.
Step-by-step explanation:
The given sentence, 'First, make sure you preheat the oven,' is a simple sentence. It is a simple sentence because it consists of one main clause, which is 'make sure you preheat the oven.' This clause expresses a complete idea and can stand alone as a sentence.