At the very beginning before Ralph knows anything about the island, his reaction is to walk around, remove his clothes, and go for a swim. He does not seem panicked or worried, but he is living really "in the moment." Even when Piggy starts to worry and ask questions about adults and getting home, Ralph seems to ignore the pressing situation he is in, and explores the island and behaves like a little boy would on a beach.
This is all very early in the novel and before any problems arise.
If you are talking about the very first reaction that Ralph has, look at the part where it is just him and Piggy. This is before they use the conch to call the other boys.
In this part, Ralph is pretty happy. The book tells us that when he figures out that there are no grownups, he is overcome with the "delight of a realized ambition." So, in other words, he is happy that something he has always wanted has come true. After that, he just plays around for a while, swimming, mostly. He's just having fun because there are no adults and no rules.