just dump out the water and clean the tank really well. use white vinegar to clean the tank as opposed to bleach.
you need to get ASAP:
a basking dock
a water heater
a UVB lamp AND a heat lamp.
a good filter
later on you will want to get a larger tank.
check out the link in the nutrition section for "turtle jello shots" they're very healthy for turtles!
also check out the indoor set up section for ideas on how to set up your tank
good luck! you're a good person for helping this turtle. can't wait to see pictures!