I've heard vitamin B12 is hard for vegetarians to get enough of. Should I be getting supplements for it? If so, do I need to make sure it's not from animal sources, or does it have such a small effect on the meat market that I shouldn't worry about it if I'm okay with drinking milk?

Also, are there any other nutrients I should be particularly worried about?

http://www.veganhealth.org/articles/dailyrecs

This site is science based, that article covers pretty much everything you might need to worry about. It is for vegans but most of it will apply to vegetarians as well.

Also I believe all B12 supplements are vegetarian, and it has been tested at massively higher than recommended amounts with no ill effects. And it is cheap! Hope this helps.

I've recently been curious if I, as a dairy-drinking vegetarian, could be deficient in B12. I don't think so, but I don't know how to check.