It is best to introduce JROTC by stating what it stands for. The reason behind its creation. The people behind its creation. Its proposed purpose and effect when implemented. The gradual changes within JROTC for the past 100 years and its effect in the future.
Basically, you just input some important aspects of JROTC in the introduction and then explain in full what those aspects are in the following paragraphs that will be the body of your essay.