There used to be a really good tutorial for this. Well click search (which is in the colored bar on top). For best result click on "advanced search" where you can manipulate different search fields such as forums, number of replies, etc.
If it's a broad subject then it's better to click on "list results as threads". Not so popular subject or if the response you need to read is part of a mega-thread, then "list results as posts".
HTH