Below are only my personal advice:
1. if we are in trouble how do we get help?
Japanese go to police boxes when they are in trouble. You can easily find police boxes in central Tokyo. In Japanese, police box is "Koban (交番)" Policemen will help you at Koban.
If you need to scream for help, the phrase is "Tasukete (助けて)" which means help.
2. what sentences should we know?
Knowing simple phrases will help you a lot to get along with local Japanese.
Such as "thank you" => "arigato (ありがとう)", "excuse me" => "sumimasen (すみません)", "please" => "onegaishimasu (おねがいします)", etc.
3. how hard is it to talk to other locals?
It might be hard to fluently communicate in English with local Japanese. (However, Tokyo is not bad comparing to other suburbs in Japan) Try to speak slowly so that they can understand. Use simple phrases. English education in Japan is a lot more focused on reading and writing rather than listening and speaking. Bring a notebook and pen to write down what you want to say. Most Japanese understand more when it's written.
If you are interested in meeting locals, Hub Japan offers a service to have dinner with local Japanese:
4. what are things we should and should not do.
Following general cultural code in public is nice. Don't speak loudly, consider personal space, don't speak on the phone when you are on public transportation, be polite, ask for permission when you take pictures, consider about people around you, etc. Trying to understand the cultural behavior is generally considered as a nice behavior everywhere.
5. how do we get help with food menus and street signs?
Google translate app have camera feature which helps you understand what the sign says. Many street signs in sightseeing destinations in Tokyo is written in English now, so it should be fine when you explore Asakusa, Shibuya, Ginza, etc. Google map will help you locate the town. The food menu is harder. There are some places have "We have English menu" signs outside of the restaurant so maybe choosing those places is a good idea.
6. where are places we should/ should not go.
You should explore Tokyo according to your interests. There are so many attractions in Tokyo, and it is various. Some people enjoy going around tourist sites such as Sensoji-temple, some enjoy a day in the park, some enjoy going to museums, some enjoy shopping, some enjoy bar hopping, some enjoy eating, etc. Personally, in my opinion, food and bars (Izakayas) are great in Tokyo so list up what you want to try (like conveyor belt sushi, tonkatsu, tempura, mochi, etc.) and try to find the best restaurants for each kind.
All places are generally safe in Tokyo even at night. Maybe walking around Kabukicho after midnight alone is not a good idea. Other than that, just follow general warnings such as don't walk dark streets, etc.
7. any warnings on what to watch out for?
Adding to the previous warnings, hmm. Maybe you should be aware that some restaurants are not foreigner-friendly. Because they have a hard time communicating in other languages, some places say you can't enter the restaurant if you don't speak Japanese. There are some "locals-only" restaurants in Tokyo exist. Some places don't take a reservation for foreign tourists. This is because some of them don't show up even if they make a reservation and don't pay the cancellation fee. Some luxury restaurants (such as Sukiyabashi Jiro) only take reservations via hotel concierge.
As I said, Tokyo is generally safe, and you don't have to worry too much even if you are an only-females group. Enjoy your time fully in Tokyo!