Hello Misfortune.
I think we'll need a little more description to solve it but here's my reply which might or might not fit.
For a basic select you have:
SELECT * FROM forum WHERE bid = $bid ORDER BY id ASC
What I think you're trying to do is to pull information into the response from a second table. To do that you need to do a JOIN. Now it depends what MySQL you're running but this is the kind of thing you should be looking at:
SELECT forum.*, poster.* FROM forum JOIN poster ON (forum.fid = poster.topicID) WHERE forum.bid = $bid ORDER BY forum.id ASC
The thing to note is that the field references are now preceded by the table name. You also need to ensure that you have two fields (one in each table) that allow you to connect the two tables - this is the relationship.
Please do re-post if I've not hit this particular nail on the head
Regards,
David