A good subject.

I'm 15 but I want to teach our church's youth group. What would be a good lecture? Thanks

Give everyone 5 pieces of paper. Ask them to list the five most important things in their lives. Most will have G-d, family, friends, a pet, school, etc.
have them sit in a circle and place a wastebasket in the center. Have them go around and each one of them must throw something in the wastebasket. They have to make the tough decision. after they do this four times have them display what is left. Many will have saved the paper with G-d.
At that point talk about how they didn't have to select between G-d or their family, etc because G-d is always with them. They can't throw G-d away because He does not throw them away. No matter what, they can turn to him, even if they have not for years...

Responsibility is always a good topic for youths (and even a lot of adults who never learned it).

Salvation, Truth, The purpose of Jesus' Death on cross are some good and important topics.

God Bless you in your work

lol im glad you want to teach. thats good. but you shouldnt really be asking this question if your a believer.

pray about it. and God will eventually reveal it to you in thought what you should teach. you believe because of what you know right? so you should have tons of things you would like others your age to know aswell.

if you dont know, then why believe, right? so im sure the Lord will reveal it to you sooner or later.

start off with the basics.how earth was made.

how to be saved romans 10:9,10