You're looking for sin() and cos().

Stick this in the Update function of an entity.

` //Math functions operate in radians, but it's sometimes easier to understand degrees.`

//Multiply any degrees by this handy constant. Also RAD_TO_DEG for the opposite

float angle = 90f * Util.DEG_TO_RAD;

float dirX = (float)Math.Sin(angle);

float dirY = (float)Math.Cos(angle);

float speed = 5f;

//now that you have a direction, you need to add it to the position each frame.

AddPosition(dirX * speed, dirY * speed);

// You can also do this by SetPosition( X + dirX * speed, Y + dirY * speed ).

//Fortunately Otter has a lot of little shortcuts like this to make things easier.