Hi Becky,
There is probably a blockage in the U-Bend under the sink. You should be able to clear this without any tools. Just unscrew the 2 rings holding the U-bend on.
If there is no blocakge there then it is somewhere further down the pipe and you should be able to tell after removing the U-bend. (keep a bucket underneath when doing this).
I hope that helps,
VJ
Vijay Rangari, February 2005