See sample prm-file as well as topology xyz-file and example of LAMMPS MD input/output as zip-archive.
The structure of prm-file is as follows (see LAMMPS manual for keywords):
# comment line
any LAMMPS input
special_bonds lj 0 0 1 coul 0 0 1 (1 1 for Dreiding, 1/2 5/6 for AMBER, 0 0 for CHARMM)
type mass class symbol coordination charge description
mass 1 1.008 # 1 H 1 0.1475 HC-alpha/beta THIOPHENE
mass 2 31.972 # 2 S 2 0.0090 S polyTHIO centr-HR
...
pair_style lj/cut/coul/cut 10 10 (refine parameters and consider other coul styles)
i ≤ j epsilon sigma (here ij are types)
pair_coeff 1 1 0.0300 2.4200
pair_coeff 2 2 0.2500 3.5500
...
bond_style harmonic
id K bondlength i ≤ j (from now on ijkl are classes)
bond_coeff 1 367.0 1.082 # 1 3
bond_coeff 2 367.0 1.082 # 1 4
...
angle_style harmonic
(it is angle between ji and jk, so that j is central atom)
id K angle i j k (i≤k)
angle_coeff 1 60.0 91.00 # 3 2 3
angle_coeff 2 60.0 119.00 # 1 3 2
...
dihedral_style opls
(dihedral is angle between ijk and jkl planes, so that jk is central bond)
id K1 K2 K3 K4 i j ≤ k l
dihedral_coeff 1 0.000 7.250 0.000 0.000 # 3 2 3 1
dihedral_coeff 2 0.000 7.250 0.000 0.000 # 3 2 3 3
...
improper_style harmonic
(improper is angle between ijk and jkl planes, i is central atom bonded to j,k,l)
id K angle i j ≤ k l
improper_coeff 1 2.2 0 # 3 2 4 1
improper_coeff 2 2.2 0 # 3 2 4 3
...