Do i need to get the first one

if i get factions do i need to get the first one too

No. Each campaign is complete in itself. You create a character and follow the storyline to the end.

There is an overall storyline that is completed in Nightfall.

There are certain skills that are only available in certain campaigns so if you don't own the correct campaign, you can't get the skill. IE, Meteor Shower in Prophecies

No, you do not need Prophecies to be able to play Factions (or Nightfall, for that matter). They are all stand-alone games that run through the same game client.

However, you do get some extra perks if you link more than one chapter into your playing account; such as access to more chapter-specific skills and the ability to make your characters travel between continents.