For rms, you have:
http://en.wikipedia.org/wiki/Root_mean_square
And for further theory, you also have the link in my previous post.

But you will normally not know the individual frequencies amplitudes and phases of the sine waves forming the complex current and voltage curves. So you will have to measure current and voltage at a high enough sampling rate and high enough resolution and then use the discrete form, i.e. sum instead of integral, to compute your Urms and Irms. At the same time as you compute Urms and Irms, you also accumulate U*I.

This will in the end give you both the apparent power and the effective power, and the quota will be the power factor. Finding the strongest current compared to the rms current will also help you with the crest factor.

The discrete formulas will just require addition and multiplication while processing n full periods. Then you compute the square roots to produce a result before starting to accumulate information for a new displayable result.