You can create an online tree but...
You will find a stand alone genealogy program desirable for a couple of reasons, manipulating data and printing reports are the two biggest I can think of. Also, you don't have to be online to work on your genealogy.
Any software program will do- you aren't limited to Family Tree Maker. Some use FTM because it syncs with the online trees while others wouldn't touch it with a 10 foot pole.
If you decide to use a stand alone package try as many as you can before you buy- make sure the program you get it the one you like most- not the one most recommended.