It means that you have made a decision about something, for example, if we take your sentence, "I want to go to Italy", now, before your decision, there was another choice, lets say Britain, your friend tells you all about the wonderful places to see in Italy and about the delicious food you can eat, this makes Italy more attractive and so you decide to go to Italy - YOU HAVE MADE UP YOUR MIND that Itlay is the place that you want to go to. I hope this answers your question.

I have just decided that I want to go to Italy. It is used if you are considering two or more possibilities and definitely decide on one them.