Standard Deviation - Equation with measured variables

My problem with math is that I don't know how to start problems. I have two that I'm unsure how to attack.

Find the standard deviation for the following equations, assuming all of the variables in them are the results of a number of measurements and therefore have error associated with them.

x=1/2at^2 in which a and t are measured values

and

V=4/3(pi)r^3 in which r is a measured value... I put (pi) in place of the symbol.

I don't want just the answers, I'd like to understand how to do them. Thanks.