Sometimes when you sing, your S, C and T sounds can be really harsh. A little too strong. In many stages of mixing you can reduce the hiss on vocals in various ways. If you are still experiencing problems with this after using the EQ and compressor you can add the De-Esser function. You can find this in the FX plugins section > Dynamics > De-Esser
There are two default settings available for female vocals. You can always adjust it to your liking.
However you can also get rid of the hiss using EQ or compressor in earlier stages of mixing. I personally have not played around with any of these options, but have run into these two tutorials on YouTube that I thought were pretty interesting.