Charles the court wrestler goes to talk to Oliver in the first scene because he is concerned about Orlando's fate. Charles knows that Orlando has signed up to wrestle with Charles, and Charles also knows he cannot take it easy on Orlando because Charles needs to establish his reputation before Duke Frederick. Hence, Charles is basically asking Oliver to warn Orlando of the danger and to discourage him from wrestling Charles.
Shakespeare includes the conversation between Charles and Oliver for a couple of different purposes.
One reason is that Shakespeare conveniently uses Charles to inform his audience about the important back story surrounding the play, which is that Frederick has just usurped his older brother Duke Senior and driven him into exile. Charles seems to be the perfect character to tell the audience about the usurpation because, despite being the court wrestler who fights opponents to entertain the court, he seems to have a rather complying nature. He doesn't seem disturbed at all by the fact that he is now under different government due to mutiny. Charles's own compliance towards the new government also further helps to characterize Oliver because Oliver is just as ready to yield to the usurpation as Charles. We can especially see Charles's compliance in his bland choice of words to describe the usurpation and exile. Since Charles uses no adjectives or figurative language to describe the act of treachery, we know he is neither feeling incensed about Frederick's actions nor does he support Frederick but rather shows he feels indifferent concerning the matter and will readily comply with either government. We particularly see Charles's bland description of the usurpation in his lines:
There's no news at the court, sir, but the old news: that is, the old Duke is banished by his younger brother the new Duke; and three or four loving lords have put themselves into voluntary exile with him. (I.i.98-102)
In addition, Charles's apparent unconcern about treachery serves as the perfect backdrop for Oliver's own treacherous deeds, specifically trying to trick Charles into killing Orlando's brother Oliver in their upcoming wrestling match before the new Duke Frederick.
Hence, one reason why Shakespeare has Charles come and speak with Oliver in the first scene is to present the back story of treachery that the play is built around. The other reason why Shakespeare includes Charles's conversation is to use Charles's compliance concerning Frederick's act of treachery towards Frederick's brother to serve as the perfect backdrop to characterize Oliver's own treacherous mindset.
Charles, although being a ferocious wrestler, appears to be good at heart. He goes to Oliver to ask him whether it is Oliver's brother he is supposed to fight the next day. When he gets the affirmative response, he asks Oliver to persuade Orlando to forfeit the match because if he does not, Charles will have to defeat Orlando to save his reputation, the result of which could be fatal for Orlando. He does not want to see Orlando harmed unnecessarily, thus he meets Orlando to discuss over this matter.