selecting sin and sin-1

I assume you mean on the calculator here.

Selecting sin should be pretty obvious, just click the sin button.

To get sin ^-1 you can either

1) click 2nd then click sin

2) click and hold sin until it changes to sin ^-1 then release

I am not sure what you mean either, but,

If you are looking for a side length you would use sin

If you are trying to find an angle then you use sin ^-1