[Legolas]

Going the longer manual way round, it's a case of typing
"mount c c:\" to get access to your C drive. You can use most of the old DOS commands to navigate. Useful ones include:
"C:" to switch to your C drive
"cd 'foldername'" to access a folder (if it's nested, either use a few times in succession or string them together, e.g. "cd 'folder1'\'folder2'\'folder3') all without the "s and 's of course.
"cd\" or "cd .." to go back to C or back one folder level
"cd\'folder4'\'folder5'" to go back to C and to a new folder directly.
"dir" or "dir /w" or "dir /p" give you a listing of files and folders in your current folder (as single column, multiple column or single column with pauses so you can see whatever scrolls off-screen).
Long file and folder names will be abbreviated with ~1 and that's what you type to open them.
"filename" or "filename.bat" or "filename.exe" will run a .bat or .exe file, which is what usually starts a game.
"exit" quits the program.
CTRL-F11 and CTRL-F12 slow down and speed up DOSbox cycles so you can adjust game speed.

Depending on the game you're trying to run, you may have to mount your D or cd/dvd drive as well. You might need to fiddle with DOSbox memory settings, fix sound issues and change game .ini files.
"mount -t cdrom d d:\" works for the former if D's your drive.

Of course, once you get everything going you get to the real issue; manuals were actually decent back in the days and contained a lot of game and background information. In some cases they were vital to completing the game. Jumping straight in might leave you feeling lost both in objectives and controls (no tooltips either to identify the different buttons).
Some games are more intuitive than others, some come with a tutorial, but in general it's a good idea to grab and read a copy.