Compute angle/local only outputs angles you have defined. If you
are asking how to define angles in the first place, that is the job
of a builder which you convert into an input data file for LAMMPS that
lists the angles. Again there are many options to building a molecular
system. See the LAMMPS WWW pages for pointers to tools (included
with LAMMPS) or other packages which do this, e.g. CHARMM, etc.
Steve