Now go to the top left options drop down and select sculpt mode. Choose one of the sculpt brushes and start sculpting. There's a mask brush, you can hide parts of the mesh by selecting polys in edit mode and hitting the h key, alt h unhides. Additionally you can create selection sets in the face maps option in object data properties.

You can control the brush falloff radius and strength by right clicking in the workspace. There's a ton of options to do this, this is simply the quickest method while working.

If you click the brush dropdown at the top of the viewport you'll be presented with additional options including various auto-masking options, which is very useful if you're doing head morphs that include head parts actors such as eyes and teeth. You can choose in this way, in a very immediate fashion, what polys are being effected by your brush strokes. Often more expedient than masking or hiding. You'll also see dropdowns for texture, stroke, falloff, and cursor. It's worth the time to explore these. The brush symmetry options are the little butterfly icon at the top right of the viewport. Obviously there's many more options but I leave that for you to explore.