If you've only got 30 mins you should try the 'body pump' program.
http://en.wikipedia.org/wiki/Body_pump
You could probably get away with using tins or something if you don't have dumbells.
A nice strength and cardio workout. I do longer versions of it at my gym and fuckin love it. Once you get doing it everyday you start noticing your strength really builds, it's not the quickest way to lose weight (running is!) but it does help.
But kudos to you for actually doing something, im sure there's tonnes of lazy assed fuckers surfing by who complain about their weight but never do shit about it except cram another mars bar down their throats..