Dear animaidi, as Code Master said before me the web site is a good start for you. A few words for the cities I've been to.
Paris is a vast city, you can easily get lost and I also didn't find many people that speaked english. French people are generally very polite though so you'll get through.
Amsterdam on the other hand is much smaller and a kind of darker city. I love Amsterdam a lot but for a person that doesn't go out at night a lot I would suggest that you should be careful... The dutch are very tolerate and open to new ideas. Amsterdam [and Berlin as well] have get parties and places to go!
Vienna is one of my favourite! Small, clean with a lot of monuments, palaces and parks. Crime rates are very low and the city is usually very safe.
Venice, Florence: Great architecture and a lot of art! A must-see for the lovers of the good arts
Rome: The city is a big museum. Everywhere you go there is some ancient or medevial monument to see. A lot of tourists! Don't forget to see Vatican City museum is a must-see for the ones that like art.
Athens is my city so I recommend it! Besides Omonoia Square [the central square of Athens] where many junkies gather, all the other places are safe day and night. As for history, I think you know everything from school
. Greeks like to party a lot, sleep very late at night, and almost everybody up to the age of 30 speaks good english [older people are having trouble with english]. The greek islands are a heaven on earth, but are numerous so you need to pick wisely where to go to [you can ask for info if you wish.
Sorry for writing so much! have a nice trip, and for the number of cities you want to see you need a lot of time! See you..