Every time you go up a level you earn at least 1 skill point. These are used to unlock bigger and better tricks; the better the trick, the more points you earn in Freestyle.
At around level 7/8 you start earning 2 skill points per level. Level 10 tricks require 4 skill points to unlock, as well as the previous trick unlocked at level 5.
As long as you have 1 trick unlocked at 5, when you get to level 10 you should have another trick available to unlock. My tip is to keep the 2 skill points you earned at level 9 and use them when you hit level 10.
Achievement pops as soon as you buy the trick