You don't really need a fleet if you are just starting out, but the purpose of fleets in NF...is 1) allow users to recive help from other players with more ease 2) HA/FW/FL/Friendly Battles 3) Make the game much less lonely.
Finding a fleet in NF is 1st come 1st serve generally (as in what ever fleet invites you first will much likly be your first fleet), unless you are a more experienced/been around your server the longest.
I play on Kaiser and some of the main fleets on Kaiser are:
But, none of those fleets really appeal to me personally, not that they are bad fleets.
I like the smaller fleets, the ones that have less member because its much more friendly. However when does a fleet become too small? To me, when you are online alone most the time its not much of a fleet; or when you can't get a hold of the fleet leader.
Anyways, to be more direct, the fleet you join is up to you, people can say "this fleet is best" but, really that's just an opinion.