Open .bash_profile in your favorite editor such as vi or gedit and add these lines.

PATH=$PATH:/usr/local/j2sdk1.4/bin
export PATH

Replace /usr/local/j2sdk1.4/bin with the directory where you installed your SDK. You can test it by running ". .bashprofile" or "source .bash_profile" (don't type the quotes) or opening a new shell. Then type "java -version" to make sure it's running Sun's 1.4 runtime and not Kaffe or Blackdown. If it's not Sun's version, type "which java" and rename that java binary, then test it again.

If you want the SDK bin directory in the path for all users on your computer, edit /etc/profile as root and add PATH=$PATH:/usr/local/j2sdk1.4/bin to the file. Then you don't need the entry in .bash_profile.