General thumb of rule is, loot everything and just sell em to an npc.
Depending on your level different things can be useful. If you're below say 20, keep the jellopy/empty bottles from porings etc. You can usually sell these to other players but not at high prices.
Honestly the best class to play as a newbie would be, either an Assassin or Swordsman. Both take little to no damage and can kill quite quickly with high strength so you can loot more things.
Also, make a Merchant, store all the items you get, level your merchant high enough for level 10 discount 10 overprice and 10 vending. Sell items to NPC's on a merchant and off you go.
If you choose to come to Rebirth, I could probably help you out alot more. Giving you leveling tips, some begginner gear, what to do, how to make zeny etc.
Rebirth is arguably the most popular server.