Get this by winning the 125cc Championship in Career Mode, which is possible in the 1st season, and will unlock at any difficulty level. Furthermore, if you make sure to win all the races, you will also unlock the 'World Champion', 'Technical Rider' and 'King of Spain' achievements. You do not have to win all the races to get the last 2, but doing so will give you more options for teams in later seasons and more upgrade anyway.