"I don't give a shit about you" is a very rude sentence. You would only say this to someone you absolutely hate (or if want them to believe that you absolutely hate them). It's a very intense, intimate and serious thing to say. I means I care about you less than I care about poop. It's hateful. Don't say this if you want to continue to have a relationship with someone. It's also probably too intimate to say to someone who you don't know very well. This is kind of a dangerous thing to say, because the meaning is so strong. If you say it in the wrong context, you might regret it. I would stick to saying something less rude and less intimate until you feel more confident about when is the right time to use this sentence. Your second sentence isn't correct. You could say, "I'm not interested in you" or just "I'm not interested". This would be a more polite thing to say to someone you don't want to date. If you want to be just a little bit rude (or dismissive) to a stranger or someone you know a little bit you can say "Go away. I'm not interested. You're annoying me." or "Leave me alone."

It is used to express your indifference. Also, to not be interested in or worried about something or someone. For instance: My ex will be at the party with his new boyfriend, but I don't give a shit. I don't give a shit what my ex thinks.

How nasty!