Firstly - install bash via ports or packages as mentioned above. Then, if you want to change your default login shell to bash now that you installed it, you can do a
to change your default login shell.
On this note - many will recommend not to change your login shell to something that is not in base, meaning to something that was not there already when you initially installed. I personally have shot myself in the foot by setting my login shell to bash, then at some later time removing or breaking the bash package during an upgrade. This lesson is especially painful when administering a remote machine without easy physical access.