That's not an easy question to answer. One way is to give the spring an instance variable with how fast it should launch Sonic (for example, "launchSpeed"). Then if you're using the Platform behavior, you can set Sonic's Vector Y to "launchSpeed*sin(spring.angle)" and his Vector X to "LaunchSpeed*cos(spring.angle)" (you might have to play with minus signs to get those equations right).
If you use this method, it will be important to set Sonic's deceleration relatively low so you can really see him move at an angle. Also, you will need to shape the spring's hitbox a special way or find some other method so the spring only triggers when Sonic hits the top and not the bottom or sides.
But overall, making a Sonic engine on C2 isn't very easy. There's one somewhere in the forums that works well, but if it were easier, I'm sure we would see tons of Sonic or Sonic-like games on the arcades. That said, don't let my nay-saying stop you.