Yamaha Blaster problems?

I just bought an Atv (Yamaha Blaster) and I am having some problems.... so when you in neutral it revs like a beauty but then you go into first the whole engines bogs down and doesn't have no power.. barely enough to pull me.. then when I am trying to go up hill it quiets it cant handle it but sometimes the engine gets its power and I am going mad fast....HOW DO I MAKE IT STAY LIKE THISS!!