I have a hiller beginner and everything seems to work right except for the tail.
It washes out everytime i spool up the heli and is never stable.
I tried moving the rudder to the extremes but it is not stable at any of the ends and pirouettes on the ground not letting me to push it forward to fly up.
Can anyone please help?

Also i have a hk401b gyro but i am not sure what value should i keep for it.

If it's pirouetting fast, then you should check if the Gyro is compensating in the correct direction.

If it's slow - Increase Tail hold gain

Make sure you are in Head Hold mode (Solid red led on gyro)

If it's a belt driven helicopter - make sure belt is tight and is not slipping.

Make sure Servo selection on Gyro is correct. Digital or analog.

Later if you get slow wag and cannot be corrected by gain then check tail servo horn linkage placement. It shall be 7 to 10 mm as far I remember for a 450 heli.