Fetch body of post?

Hey all.

I am making my own little plugin, but I need some help.

I would like to get the body of the first post from a given topic_ID. I have looked in the functions database, but I don't know which one to use.

My guess is that I would first need to somehow find which post_ID is the first one to link the given topic_ID? and once I have the post_ID I'll call the appropriate fetch_function to get the body?

Does someone know how to do this? All I need is a direction of which functions I need to use - I'll hadle it from there on.

Thanks!