Where are you starting from?
Go to any of the airline purchasing sites, and find out what a ticket is going to cost.
check the hotels you think you might like to stay at (a "moderately priced" hotel can run a couple of hundred dollars a night). Add that all up.
You don't even say how long you plan to travel!!! That is key!
How do you plan to travel between cities? Trains? Planes? do you want to buy a rail pass? Remember, that doesn't cover everything: You need to purchase seat reservations, and often surcharges for the more popular routes, beds, if you plan to sleep on them...you can't just fold out the compartment seats, and sleep, six to eight strangers in a compartment anymore!
Plan a day for each city, just to travel.
Finland and UK are going to be more expensive than France and Ireland, but I think you have picked four of the most expensive countries in Europe!
Only you can do this...
Once you have all the dollar figures (I'm guessing you're US, since you mentioned "high school".), add 20% to cover inflation and currency fluctuations. There, you will have a pretty good idea what you need to have.
When planning your time, remember, you probably have to be back for university before the second week of August, and you'll need to do tons of shopping, getting your wardrobe and dorm equipment prepared. That takes time.