Ditto on the traveler's checks - just use your ATM card, as you'll get the best exchange rate that way. Keep in mind that if you use a credit card, your bank may charge an additional percentage for overseas purchases.
Getting from the airport to the hostel in Paris: Get to the RER (local train) station at Charles DeGaul Airport. You will likely take it to Gare du Nord or Chatelet/Les Halles station. Check with your hostel to see which is closest. From there, you'll probably have to get on a Metro (subway) train. As for passes, it depends how long you plan to stay. you can get a one-week Paris Visite card at a decent price, or if your stay falls within the right timeframe, you can get a weekly Orange card (carte hebdomadaire).
Getting to Berlin: Look for "white" fares on the trains. YOu should be able to check out the schedule at any train station (i.e. Gare du Nord, Montparnasse, etc.) - the RER station at the airport should also have a kiosk or two with all the train schedules and ticketing. Take advantage of any youth/student discounts you can find. Alternatively, there are tons of cheap flights in Europe. Check out whichbudget.com to find out the various carriers from Paris to Berlin, and you might find that it costs less than the train.