So yeag, I bought Silph scope and went to both Ghost town and Lavander tower. The description about this item says it allows you to see rare ghost pokes but I think I didnt see anything rare, what rare pokemons does it mean? And while on the maps, I found a treasure box and got a Dragon scale? What's it for?

Sliph scope is for the ghost on the top floor of Lavander Tower, once you used it the ghost there will turn into ShinyMarowak (like in the red, blue and Yellow ,Green game system) one you battle it and wins, the shinymarowak will disappear and leave behind a "strange note", so far no one know what that does.

Dragon Scale is used to evolved Seadra into Kingdra.

I don't know how to use it. Shall i equip it with my starter pokemon or it will just work on its own? Btw thanks!

Goto the end of the map and there should be a Ghost. Walk up to it and battle it and when you are in battle, click on Backpack to use it.