[lammps-users] unit cell and simulation box

The doc page for the lattice command explains all its options.
The custom option is the most general and you can define
lattice vectors (if that is what you mean by a,b,c) or lengths
or an orthogonal unit cell (if that’s what you mean). I suggest
you read it carefully, generate some lattices of atoms and visualize
them, starting in 2d, which is easier.