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If it's at a party or dinner, a lot of time people don't say aloud that they need to use the restroom. Often it would simply be, "Please excuse me for a moment, I'll be right back." "I need to step away for a moment" and it will be kind of understood. You can use these lines for using the restroom, making a phone call or smoking.
"I have to go" is not appropriate unless you are a kid. As an adult it could be misunderstood as you need to leave the party/meeting now and go somewhere else, and people mayart saying good bye to you. When you say "I need/have to go to the bathroom" it implies a sense of emergency.
If you are asking for directions to the restroom, then you can ask "where's the bathroom" if you are in a house, or "where is the restroom" if you are in a restaurant. In the latter you can also say "men's room". A polite way to say it, if you are in a fancy restaurant or an office, would be something like. "Excuse me, would you mind showing me the way to the ladies' room/men's room?" For ladies, an even classier way is to call it "the powder room", that is, you are going there to touch up on your make up instead of tending to a call from nature. You can use the term when you go to an expensive hotel/restaurant.
Toilet and lou can be used but are less polite, so it's better to use restroom/bathroom if you are not talking to friends.