Hill climb is probably the hardest part of the game. when you start make sure you keep your finger on the down button so that you do a slight wheely at the start to give you an advantage. you will usually see a dark trail of mud leading to the top of the hill.
The easist way is to take that route. its not easy and will probably take you a few tries to even get to the top never mind winning but you will soon get the hang of it.
Freestyle is my favourite. When trying to get the high score just take an easy way going around in a circle of one jump and then turning to go off the next. its that easy. try to do combinations of 3. when doing tricks make sure sometime hold the stunt modifier button to mix up the tricks that you are doing,the score will go down for a trick that you keep doing!
By winning races and coming first in freestyle and hill climb you will unlock new gear for your rider and new bikes !
Note that this walkthrough is not for the atv's as i have not had a go at them yet...sorry !