Installing LaTeX on iMac
TeXShop is already included in MacTex (also LaTeXiT, which is probably more interesting for you). Basically, MacTex is a carefree package, which should include everything you would ever need in a  LaTex distribution. This really is the best way to install LaTex on a Mac, it's easy and requires little to no configuration (basically what you get used to on a Mac).

If you just want a text editor and run all the LaTeX command in Terminal, you can use TextWrangler, which is free and very good. It's not LaTex specific though and doesn't do any LaTex highlighting. If you want that, you may want to look at Aquamacs Emacs, which does very good highlighting.