Taking a logarithm is just the oposite operation of making something the exponent of something else (I worded it like this on purpose).
consider
a
If I take a and make it the exponent of b
b^a
I can then take the logarithm of base b of b^a to get a again
log{b}(b^a) = a
Try not to memorize specific rules like they're probably teaching you ("if you have a logarithm of this, then rewrite it like this"). That really doesn't help, and you won't remember it by the time finals come around. Just sit down and think about the concepts for a while, eventually it should *click* and you'll understand what you're doing.