Hey! I love this question. If you're not sure where to start, ask your parents or preist! they, of course, know mostly everything about the church. The most important part about catholocism is that God sent his son Jesus to be a perfect man and die so that we could live and eventually go to heaven with him

that's the basis of every christian church. catholocism has several beliefs that differ from other christian churches and here's a website that sums them up pretty well.
http://www.catholic.org/prayers/beliefs.php
One difference between Catholics and other christians that is pretty important, in my opinion, is that Catholics rely not only on the bible for teachings, but also on tradition! which is why the mass remains how it always has been. The bible tells us to follow tradition (2 Thess 2:15) so we do!
Catholics also believe that the eucharist is truly thr body of christ, because he told us the bread he broke is his body.
Sorry this is so long, hope it helped! I will pray for your friend!