Even under the sink the rules of righty tighty and lefty loosy still applies to the sink trap. :-)
Just assume you are facing it, so figure out which is the top and assume you are facing it, and turning right tightens it. On my sink I just checked it, and on the front of the U and back of the U shaped pipe, both bolts are facing down, so either one of them facing from the top would turn "left" to loosen it.
Dont forget to put a bucket or pan under it so you dont get water all over your floor, the trap will be full of water when you remove it :-)