The general equation for position of a particle performing SHM is of type

$x=A\sin(\omega t+\delta)$

Let initial position be $\alpha$, therefore

$\alpha=A\sin(\delta)$

Let velocity of particle at $x=\alpha$ be $\beta$

$\beta=A\omega\cos(\delta)$

Now you have two equations and two unknowns. Solve them and you may find $A$ and $\delta$

The general equation for position of a particle performing SHM is of type

$x=A\sin(\omega t+\delta)\tag{1}$

Let initial position be $\alpha$, therefore

$\alpha=A\sin(\delta)\tag{2}$

Let velocity of particle at $x=\alpha$ be $\beta$

$\beta=A\omega\cos(\delta)\tag{3}$

Now you have two equations and two unknowns. Solve them and you may find $A$ and $\delta$