  We studied the effects of nonmagnetic impurities on high-temperature
superconductors by solving the Bogoliubov-de Gennes equations on a
two-dimensional lattice via exact diagonalization technique in a fully
self-consistent way. We found that s-wave order parameter is almost unaffected
by impurities at low concentrations while $d_{x^2-y^2}$-wave order parameter
exhibits a strong linear decrease with impurity concentration. We evaluated the
critical impurity concentration $n_i^c$ at which superconductivity ceases to be
0.1 which is in good agreement with experimental values. We also investigated
how the orthorhombic nature of the crystal structure affects the suppression of
superconductivity and found that anisotropy induces an additional s-wave
component. Our results support $d_{x^2-y^2}$-wave symmetry for tetragonal and
$s+d_{x^2-y^2}$-wave symmetry for orthorhombic structure.
