Carl Granderson sentenced to 6 months

Does this mean he will be on trial next week?how can he be ordered to six months of jail if the agreement was rejected?
It's a matter of state law, but in general sentencing is left to the court. The prosecution and defense can reach an agreement on regarding what the defendant will plea (i.e., guilty) and what charges will be brought (e.g., battery, but not a more serious charge). The deal can make a recommendation as to sentencing, but the judge is not bound by it. Typically (though it may vary by state) a judge will explain this to a defendant before the plea is entered. Basically the judge will say something along the lines of "you understand that while you are pleading guilty and the prosecution is recommending sentence X, I am not bound to impose sentence X and can, in fact, impose a greater sentence."

As a general matter, it is rare for a judge to do this, the criminal justice system depends on pleas deals being reached and entered without much fanfare. If a judge frequently rejects the terms of the plea deal, defendants before that judge stop entering pleas and the judge's docket can be overrun. Then again, this may have been a high profile case in Wyoming and the judge felt the need to be seen as tough on crime in order to bolster re-election chances.