I am afraid it's not enough to mention it's (slang!!!). Реально is used mostly by teenagers, в натуре is used by criminals. Valda, i believe you don't want to sound funny or impolite. So it's useful to know such words to never use them.
Use "на самом деле" - this is neutral and sounds natural. I really wanna drink - Я на самом деле хочу пить. (Я в натуре хочу пить sounds like prisoner talk). "Действительно" is a bit formal.
BappaBa, чему научим человека? Говорить на быдлячем наречии?)))