Wat happens to some1 who commited this first offence?

well my son is 15 and comitted robbery to da second and ia charged with dat and larceny i think and illegal firearm which was da bb gun he and his friend had so like he went to court july1st and dey gve him anotha court date sep16 and he pleaded npt guilty..so wat is dere to expect? what happens if he pleads guilty?and its his first offence