Hi and welcome to LQ!
The best solution is probably adding the bin dir of the Java SDK to your $PATH environment variable. The $PATH is a list of directories that will be searched for executables, and you can set it globally or just for yourself.
In the file /etc/profile you can add a line that goes something like this:
The line above assumes that the you have the SDK installed to /usr/local/j2sdk-1.4.02_03 - if you have it elsewhere you have to change it to whatever is appropriate for your installation. The $PATH is a colon-separated list, so the line above means "set the $PATH to whatever it was before plus our Java bin directory".
Then simply log out and log in again and you should be able to type javac
(or whatever) at the prompt and it will launch and function as expected.
For programs that you install manually you shouldn't use /bin or /usr/bin - drop them in /usr/local/bin since that's what the /usr/local tree is for - locally built software.