In act 1, scene 5 of Hamlet, what information does Hamlet share with Horatio and Marcellus?

Hamlet in Act I scene 5 has just received some very shocking news from the Ghost, and when the Ghost leaves and he returns to Horatio and Marcellus, they are of course very curious as to what the Ghost told him. Hamlet does his best to not tell them anything, but he make Horatio and Marcellus swear a vow to not reveal anything of what happened that night to anyone, and he does reveal something of his future plans as the following quote indicates:

As I perchance hereafter shall think meet
To put an antic disposition on...

Hamlet therefore tells Horatio and Marcellus very little of what the Ghost told him, but he does tell them that he is planning to pretend to be mad for his own purposes. He does not at this stage reveal that his father was killed by Claudius, but he does tell Horatio and Marcellus that the Ghost is "an honest ghost," rather than being a malevolent spirit, as they initially feared.

What information does Horatio convey to Marcellus and Bernardo in Hamlet?

In the opening scene of the play, Marcellus and Bernardo show Horatio the mysterious apparition that resembles the deceased King Hamlet. Horatio is astonished by the Ghost and concludes that the Ghost's presence signifies that something terrible is happening in Denmark.

Marcellus then asks Horatio why he is standing guard at night while the country of Denmark seems to be preparing for battle. Horatio responds by telling Marcellus and Bernardo that before King Hamlet died, he defeated King Fortinbras in a duel and claimed some of Norway's territory. Recently, the son of the deceased king, Prince Fortinbras, has raised an army with the intention of reclaiming his father's lost territory. As a response to Prince Fortinbras's aggression, the country of Denmark is preparing to defend itself. Horatio also wonders if the Ghost has anything to do with the country's preparations for war.