How to do this?

[eluser]mlakhara[/eluser]
You can use nested views for display.
At the first level you display the categories(using foreach) and in side this you can again load another view to display the forum topics under that category.
Also Data fetching can be don in the similar way using models for category and topics.

This way you will get the data in the form of a nested associativ array which then can be used easily with each nested view(as the associative arrays break inside the view).