I am travelling with my family ( with two kids and mother & father) to US for 30 days and first planning to land at Chicago. How best we can plan our trip.
Planning a trip properly can be a major undertaking, but it will make your actual experience much smoother. Without having an idea of what you want to see or do, I can only provide you with a broad bit of information that may make your travel a bit easier.
If you are flying in internationally, you will land at O'Hare Airport. Transportation should be your primary concern, unless you have relatives or have already had this provided for you. America is very large, and does not have the public transportation systems that are common in Europe. Travel by train is possible, though can be cost prohibitive, via AMTRAK. An AMTRAK Station is near O'Hare, but will require a bus ride in order to get there. The best way to travel in the US is by car, though travelers from other countries usually underestimate exactly how big the US is, so you need to plan your routes before you arrive. Driving from the East Coast of the US to the West Coast will take at least 3.5 days if stopping for sleep, just to give you an idea. Cars can be rented long term from several companies at the Airport, such as Enterprise, Avis, Hertz and Budget. A credit card will be required, and it would be best if you make reservations prior to arrival. An international driving permit can be obtained through AAA, and you should become familiar with US road signs. The Interstate System traverses the US, and will get you where you need to be. Interstates are marked with with blue shield shaped signs, and have names that start with I (I-94, I-55, etc..)
Accommodations can be easily found via online sites such as tripadvisor, restaurants via yelp and urbanspoon, and general advice for the country or state can be found at wikitravel. Multiple sites, including tripadvisor, can help you find things to do in each area.
Cell phone coverage is extensive, though there are several competing companies that provide service. If you have an unlocked cell phone, you can change out your SIM chip for a US chip. If not, pre-paid phones from companies such as Boost, TracFone, and Virgin Mobile are readily available, cheap and basically disposable.
Language can be an issue in smaller towns, but you will find most people patient and willing to help you. If you are moderately proficient with English, you will be fine. Spanish is also widely spoken, and many other languages are spoken by Americans of many different backgrounds. A general vocabulary of "Please", "Thank You", "Help", "Hello" and "Goodbye" will get you surprisingly far. American English is full of contractions, slang, different pronunciations, regional words and accents; so please do not feel overwhelmed.
Notifying your countries Embassy in the US of your arrival and itinerary is also very helpful. In the event of an emergency or a run in with US Law Enforcement, they will be able to assist you. Always carry your passports with you at all times.
I hope you have a great trip!