Using Plain TeX is not really recommended for the typical user, esp. if one needs to interchange w/ others, or make use of pre-made code / formats.
LaTeX is the most prevalent TeX macro package, and well-suited for most users who need math. Read over _The not so Short Introduction to LaTeX_ to get a feel for using it directly:
http://www.ctan.org/tex-archive/info/lshort/english/
Alternately, there's a very good graphical front-end, LyX:
http://www.lyx.org/ which has had some work done recently on making it round-trip w/ Word --- might be you could import the files into it and then export them from that.