Before you start porting an application please check if it is already available in HaikuDepot or via haikuports (the link has already been posted)
If it is not available already I would start by checking what prerequisites the app has, e.g. which language it is written in, on which libraries it depends, and if it is a GUI application the most important thing is the GUI toolkit it uses. QT is already available under Haiku, GTK isn’t (apart from a few experiments). After checking that you just start compiling the app and dealing one by one with the errors that are likely to occur.
You can ask for help here, or on the haiku IRC channel where most of the developers and software porters are around.
Anyway, have fun and don’t hesitate to ask for help