Install the software (X if it's not already installed plus the LXDE packages), then create an (dot)xinitrc file (if one is not already present, and it likely isn't) to call it from the command
startx.
If you are not currently booting to a graphical interface, installing the desktop package should not change that; just make sure you
don't install a DM (graphical Display Manager--that's the program that handles graphical the graphical login and session chooser) along with the desktop package.
The Arch wiki has a good article on .xinitrc files to get you started. A web search for "using xinitrc" will turn up much more.
The .xinitrc I'm using now consists of one line:
Note that, in many distros, editing /etc/inittab to boot to runlevel 3 instead of 4 will give you a command line login (3 = command line, 4 = GUI), but in Debian and many of its derivatives, runlevels 2-5 are treated the same as regards the login by default.
Here's an article on runlevels in Ubuntu:
http://www.pathbreak.com/blog/ubuntu...jobs-explained