If you are using a mirror modifier, you will need edges exactly at the mirror boundary. If you have Clipping enabled on the modifier, you can quickly duplicate any edge or vertex and move it towards the boundary, each vertex will get stopped exactly there. Lock movement to an axis if needed. If you use extruding, later you will have to deselect edges before filling.
The part to be filled has to be closed. Close it by making more edges (F or Mesh > Edges > Make Edge/Face).
Looking at the side of the mesh in orthographic projection, border-select one column of vertices (B or Select > Border Select). This should be done in wireframe view, to select vertices behind faces. Then fill the area with triangles (Alt+F or Mesh > Faces > Fill).
If this filled more than needed, you will have to retry on a smaller region. Undo the fill and make some internal edges, then try again.