Here is a list of all the DOS commands you can do under DOS. I don't think you can do more than a handfull in DOSBox tough, and they are all listed in the helpfiles.
To switch to your hard drive, let's say it's c:\
You type:
mount c c:
c:
And you should end up in your HD root directory, which is c:\
To mount CD drive or such, look
here.
For a complete guide on how to configure DOS for games, look
here.