Your main expenses will be transport, accommodation and food.
You will have to decide how you are going to get around....some people consider a eurorail pass is the way to go but it depends on how long you are staying and where you are going as some countries are not on the list or there are limited trains.
buses are reasonable value try something like www.eurolines.com i think or you can also link thru the www.nationalexpress.co.uk ( to europe). otherwise you can also get some cheap flights at times too...try something like www.skyscanner.net as it has heaps of the cheapest airlines in europe.
for accommodation sharing rooms is the cheapest ( or you can get night trains/buses and sleep on them and save time and money).
try websites like www.hostels.com, www.hostelworld.com, www.hostelbookers.com. you will note that some countries and cities are more pricey than others and you will have to decide how long you want to stay in each place . Also i found that a cheaper place is often further out of town or even in another nearby town/village. You have to weigh up to cost of travelling in and out and the time it takes ( eg i spent up to an hour each way in and out of florence to my cheap accommodation which got tiring at times).
the further east and south is generally considered to be cheaper that east/north....i can highly recomment czech republic ( prague is a lovely city and one of my favourites..also try to get to cesky krumulov, a little village which is lovely).
Croatia is wonderful and is a bit cheaper than some other coutries as well...dubrovnik, split and zadar on the coast are nice and the coastline is stunning. Zargreb is nice too.....but they are not large and do not need much time to see eg 1-2 days.
Italy is wonderful for things to see, particularly the central /north areas like verona, florence, bologna, rome etc
France has some great places too and you have to go to paris as it has so much to see and do.
Anyway do some of your research into these websites and see what it costs...try www.lonelyplanet.com..maybe buy a travelguide for europe as well as it also gives you some idea as to prices in each place.
