I know TeXshop is popular and supposed to be easy to use. I don't have a mac so cannot really judge, but friends of mine used it. TeXworks is another simple editor that runs in windows, linux and osx. I like a little more options personally, but perhaps it is a nice one to start with. It seems TeXmaker also runs in osx, out of the three I would probably pick that one.
A quick google search for texshop spell check, showed that this feature can easily be added.
Besides an editor you need a LaTeX distribution of course, for osx this would be MacTeX. There is some info on installing LaTeX in the first appendix of the lshort guide.
Another good source of information is the tex faq,