I would start with scenery too. You will see first results within minutes or hours, taxing around your very own airport.
www.airportdesigneditor.co.uk is a freeware you may want to use. The docs are very good I think and there are quite some users out there which may help.

The plane stuff is way harder, but you could start enhancing the default ones. So maybe try to add some glass to a steam gauge plane or edit some vital flying parameters to check how they influence the experience.
Make a backup of the corresponding aircraft folder and then use sources like the FSX SDK description or the Net based one here. http://msdn.microsoft.com/en-us/library/cc527029.aspx
It explains all the default values, so you can catch a glimpse on how they affect each other.