With any of the achievements in MotoGp 06 where you have to win the championship. Rookie, Professional, Champion and Legend, in both Extreme and Grand Prix modes.
know that you do not have to win any race just so long as you finish in the top three in all races, as even the top riders do not finish some races or are placed well down the field.
Obviously winning helps and you are more than likely to win all races in rookie mode and most races in professional.
The only difference with Champion and Legend is that you will have to qualify high up the grid to be placed in positions 1-3.