The GTN is the Galatic Trade Network, it's located on the Imperial/Republic fleet (your first trip should happen around lvl 10). It's (sort of) the auction house of this game, although u can only buyout items and not bid on them.
After you have completed the first chapter of your class story you unlock your legacy. Via the legacy menu you can buy all sort of convience stuff like rocket boots (speed bonus for short duration), more xp for WarZones/Flashpoints/exploring, mobile skill respec.
You can buy everything with credits or cartel coins, some stuff requires some special pre-quests..
The other big thing is the cartel market where u can buy all sorts of appearance/convienence stuff like "cool looking" armors, color crystals, mounts, emotes. The cartel market can be overwhelming to new players.
It's worth meantioning that most of the gear of this game is modable. This means for example a chest piece consists of Armoring, Mod, Enhancement (and Augment slot is you add one). You can rip these out and put in different (looking) armor and keep the stats you like. Furthermore you can add different dye's to your armor to your liking.
To summerize:
Most of the stuff you can buy is for convience and appearance. Especially the legacy stuff can grant you xp boosts.
If you don't care for things like that you don't have to spent credits/cartel coins.
Maybe someone has some tipps what worth for f2p players, I'm subscribed since launch so I don't know how bad it is for f2p. (Many convience stuff missing for f2p)
Cheers!