Need an Advice

Hello
I attend to visit Paris for the first time Next month ,,, i don't speak any French i only speak English ,,, but that didn't prevent me to visit the city of love and Fashion ,,, i can't hide my fears from getting lost but i hope i will manage it through a map and mobile phone ,, but my biggest fear is meeting thieves or con people ,,, so i will appreciate any advices you can give me to have a nice safe trip ,, is the rer is a safe transportation or i should use a taxi ,,, what time should i be at home as girl ,,, what i can do not to let thieves know that i'm a visitor
thanx iin advance