The motion of a mass tied to a string in a vertical circle includes following mechanical concepts.
It must satisfy
(i) availability of centripetal force to remain in a circular path
(ii) satisfy conservation of energy
If we take a situation that the ball just reaches the topmost position with velocity equal to zero then the the tension in the string will be such that it is just taut.
Therefore the gravitational pull must be providing a force equal to magnitude of centripetal force (m.v^2)/r so one can get the value of speed and the total energy is known.
In other cases also where the body can reach the top and can cover it with some finite velocity the total energy conservation can be applied with due consideration of change in potential energy of the body.
I will advise you to take the topmost velocity to be say v(3) the bottom point as V(1) and at horizintal midway V(2) and relate the energies K.E. +P.E. at the three points to be equal.
you have a info that at top point the only force is mg acting downward providing the centripetal force. that will facilitate with the value of V(3).
Then you can calculate v(1) and then naturally V(2) can be easily computed.
The Tension in the string at the horizontal point where the speed of the ball is v(2) T= m(v(2))^2/r as the mg force is perpendicular to the string and not contributing to the tension.