command to run another shell on top of your default shell?

Can someone point me to the right command to to run another shell on top of your default shell?

Thanks

PS If admin sees this It is not home work. I am old man who wants to learn unix in my spare time. I CURRENTLY DO NOT GO SCHOOL, COLLEGE OR UNIVERSITY.

After you login, # preceed comments and are not commands you enter.

# to get the bash shell
/bin/bash
# to get the ksh shel
/bin/ksh

When you want to quit, you will be down inside a second shell. So that means
you next to type exit <return> exit <return>. The first exit gets you out of your preferred shell, the second one gets you out of unix.

Thank you.

Can i ask you one last question.

Lets say my default shell is bash and then i load up csh and then ksh. How would i exit csh without exiting ksh?

so basically i gone from bash > csh > ksh and i wish to close csh