First thing you must do is unlock a bike. The most basic bike, 'the Baggie' can be unlocked by completing the very first training course in your career.
Then select the 'Ride' tab (LT/RT switches them) and 'Track Central'. Here you can see tracks made by the community, include 'newest easy tracks', 'newest medium tracks' and 'newest hard tracks'.
Simply choose 'newest easy tracks' and select a trials track you fancy (pick trials or FMX as these can be completed on the Baggie).
Complete the track and gain your achievement!