The first thing you have to do is to clear the blockage causing the backup. You can rent a small power snake from a local rental yard, pull off the trap under the sink and run the snake down the line. Make sure you run it more than once, then reassemble the trap and check it. As you are running the sink, drain cleaners typically use a small hose attached to the faucet to put into the opening with running water. The water helps remove the broken up sludge. Do not turn it on, though till the second cleaning.
Then I would find out why the water is coming out under the house. Is there an open pipe? Is there a broken pipe? These are questions you will have to answer yourself, by crawling under the house, or getting someone else to do if for you.