You may find Polish traditional cuisine meat-oriented and a bit fatty. The basic vegetables are potatoes, tomatoes, carrots, cucumbers, onions and most available meat is poultry, pork and beef. Of course we have accepted many influences from other cultures and you have plenty options to choose from.
Typical dining day starts with an extensive breakfast, differing from the "coffee and biscuit" ones. The main meal of the day is "obiad", eaten around 3-4PM ("after workday") with two courses and a dessert. Evening meal, "kolacja", is much simpler and often cold. Fortunately, this code is not strictly observed by restaurants. Usually you can order warm meal any time from early afternoon till late evening. Most Poles eat their meals at home, so restaurants are targeted at tourists and visitors.
Tips are (of course) welcome by waiters, but not obligatory and there's no "standard percent" to give.
Tap water should be boiled before drinking.