For a beginner whose source is Word documents, you should take a look at Atlantis, there's a thread on it here
. It costs a small amount, but you get a trial. Sigil is free and very good, but to use it properly you need to know how to code in xhtml and css, it's more of a development environment than a converter. Calibre is good as well, but the source needs to be formatted quite precisely. I find that all the automatic converters need some touching-up after processing, but Atlantis probably gets closest to a professional output.
Just stick to 11 or 12pt font for the body and make sure you use styles for all your formatting.