tips on how to build a terrarium

hello! we are grade 12 students from the philippines and our teacher assigned us to make a terrarium as a group for our final project for earth and life science subject for us to graduate high school. like it's one of the requirements since it's a major subject in our curriculum. i'm currently gathering rocks for our drainage. i read on google that we should at least give the rocks one by one a drop of vinegar to see if its a limestone to avoid the ecosystem from rotting. i'm searching online, but i also want to get tips from people who already have experience building terrariums. we are not allowed to open the jar once we closed it, so we are kind of struggling on this part.

1. what kind of living organisms should we put inside? something that can sustain itself in the ecosystem we built? something we can find, please. we are students, so we can't buy it.

2. is it okay if we get rocks from the rivers? we live in a nearby river, so we think gathering rocks there would be great idea.

3. do we need to sterilize the rocks once we obtained them, or do we need to wait until we are told to assemble our jar?

4. what kind of soil should we put in there?

other tips would be appreciated. thank you so much!