Hi, Adrain —

We can't answer this question directly because it's your job to study, read, then share what is important. What we can do is give you a good source, the Catechism of the Catholic Church, to find relevant information:

Here are the search results for community church


You may find these pages helpful for article 9: I Believe in the Holy Catholic Church.

• Paragraph 1. The Church in God's Plan
  
  • I. Names And Images Of The Church
• Paragraph 1. The Church in God's Plan
  
  • III. The Mystery of the Church
• Paragraph 3. The Church Is One, Holy, Catholic, and Apostolic
  
  • III. The Church Is Catholic; What does 'Catholic' mean?
  
  This one may be a heavy reading for a 15-year-old but it's a good source:
• Mystici Corporis Christi
  

Key points I would mention are:

• How our sacramental lives, the good works, kind deeds, and our daily prayers not only affect the individual Catholic but affect the other members of the Church.
• the down side is our sins, shortcomings, and bad behavior also, not only affect the individual Catholic but affect the other members of the Church as well.
• this is unlike the theology found in Protestant churches where, generally, there is only a
  me and Jesus relationship instead of accompanying the first bullet with a me and Jesus relationship.

Note I said generally. There are some churches that have a similar view of community as the Church does. Nevertheless, we still disagree on doctrinal issues.

A few days prior to your talk, [pray to|ask] the Holy Spirit to reveal to you what He wants you to reveal to your audience.

I hope this helps,

Mike