Either is correct, but they have slightly different meanings. "Today we are going to learn about sentence structure." This refers to the entire TOPIC. The sentence implies that you will introduce the topic and learn about only part of the topic today. "Today we are going to learn about sentence structures." This refers to specific POINTS or COMPONENTS within the topic. The sentence implies that you will teach/learn about ALL of the different sentence structures today. If the teacher and students had never talked together about the topic before, I would recommend the first sentence, because the topic is being introduced; you may or may not discuss every sentence structure. If the teacher and students have the goal of learning ALL of the different sentence structures today (which would take quite a long time), I would recommend the second sentence. If the teacher and students have spent time learning about this topic previously, and today the topic will continue, I would recommend a variation of either sentence: "Today we are going to learn about a new sentence structure." (only one) "Today we are going to learn about a few new sentence structures." (several)

They are both correct sentences, but they have different meanings.

Sentence one is the correct sentence.

Number 2 is better because you are talking about multiple sentence structures.

Both are ok but structures is better.