0. There is a "Sticky: FAQ -- Got Questions? Ask Here!" thread where you should post all your Questions: http://www.fantasicaunofficial.com/t8003-faq-got-questions-ask-here
1. (P) items are always used first before the tradable items automatically, but you can't see if you are already out of Personal items when using an item
2. (Fallen) Tonics are event items only used for Guild War events. Every new GW event, the tonics will have a different name attached to it and a different color though. Also specific for GW events are (Royal) Souls. These event items will disappear when the event is over
3. Soul Stones (S) or (L) are used to recruit certain monster cards to unit cards. They won't be deleted even after the Recruit event is over and can be used at the next Recruit event
4. The trading is only P2P, there's no bazaar or auction house or similar trading mechanisms in this game (unfortunately)
And welcome to the game and this forum
We will be your guide when you're lost.
EDIT: lol ninja'd XD