depending on the shell you use, you can
edit your .profile, .bash_profile, .cshrc,
.bashrc, or .kshrc file(s) and modify you PATH
variable in "append" what ever you like to the
existing information.

I would recommend you get a book on the
particular shell you are using (sh, ksh, csh or bash). It will prove invaluable.

example entry in .profile:

# Append the "bin" directory in my home
# directory to my system default path
PATH=$PATH:$HOME/bin
export PATH