the forces acting on the mass are tension (T) in the string and weight (mg).

weight is straight down in direction.

let be the angle the string makes with the vertical.

the components of T are ...

since there is no acceleration in the y-direction, net force in the y-direction is 0 ...

(1)

tension in the x-direction is providing the necessary net force ...

(2)

solve for T in the first equation ...

substitute this expression for T in the second equation ...

go back and find T.