You need to get control over your cell styles. I suspect you're trying to enter text into an input cell style and Mathematica is attempting to interpret it as Mathematica code.
Place the cursor where you want a new text cell to start. Then choose Format/Style/Text. Mathematica will then create a default text cell which it will interpret as just text. Now to enter an equation, place the cursor where you want it, then choose Format/Style/DisplayFormula. Mathematica then creates a cell that you can enter special symbols such as integral, partial, sums, and others.
This is under the general concept of "StyleSheets". When you startup Mathematica, it uses a default style sheets containing the style you see in the Format/Style menu. Now suppose I wanted to create a report, I can choose Format/StyleSheets/Report. This then changes the default style sheet to Report and now the Format/Style menu should contain styles reflecting cell styles associated with a report.
Personally, though, unless you want to embed mathematica code for a presentation, I would think just writing your work in TeXnicCenter latex would be a better choice.