Remember that STP stands for standard temperature and pressure.
The first thing to do is to use molar volume to convert from litres to moles.
You are given the volume of He, which is 56.0 ml. First convert this to litres. This is equal to 0.056L.
We know that 1 mol = 22.4L.
Next, take the volume in litres of He and divide it by 22.4L.
so 0.056/22.4 = 0.0025 moles of He