You have to take a bus from LGA to get to the subway, so if you haven't done it before and you have an interview to get to, a cab is probably best. If you're still interested in taking the bus/subway, there is a fourm you can search in that should already have details:
http://flyertalk.com/forum/forumdisplay.php?f=465
Without knowing exactly where you're going in 'Lower Manhattan', I'd say to allow about $25-30 (plus tip and any toll) for a cab from LGA; your actual fare depends on exactly where you're going, what route you take, whether the route involves a toll, etc.