Some of the most common commands include:
\frac {numerator}{denominator}. For square roots, use \sqrt[root]{number}.\leq gives the less than or equal to symbol, \geq gives the greater than or equal to symbol, \neq gives the not equal symbol, \sum gives the summation symbol, \partial gives the partial derivative symbol, \nabla gives the Laplacian operator, \times gives the cross product or multiplication symbol, \cdot gives the dot product or multiplication symbol, and \int gives the integral symbol.\rightarrow and \leftarrow give right and left arrows, respectively.\% as the percent symbol is normally used for comments.x^2, or, for longer superscripts, x^{2x}. To do a subscript, you can type x_a, or, for longer subscripts, x_{ab}.\boldmath{...} to make your math symbols bold. Other options are given at this TeX.SX question. Math symbols are automatically italicized; if you don't want this to be true, make your equation text as described below.\infty.\int, and then use the \limits command. An example is \int\limits_{\infty} or \int\limits^{\infty}. Then, for normal cases, you can do \overset{top}{normal} or \underset{bottom}{normal}. This can be very useful for doing vectors. For example, you might do \overset{\rightarrow}{x} The amsmath package is need for overset and underset.\{ or \} to get curly braces.\usepackage{amsmath} in the preamble, and then type \text{...}.\quad between the two items you want to separate (for example, you might have $2x \quad cos).