What's the best way to Travel Europe for a low price?

My best friend and I are seniors in high school. For her graduation gift, her dad is sending her and I to Europe. We want to go to London and Paris for sure. We also are thinking of taking trains around to other countries. We would like to keep the price reasonable though we realize this is not a very cheap trip.... show more My best friend and I are seniors in high school. For her graduation gift, her dad is sending her and I to Europe. We want to go to London and Paris for sure. We also are thinking of taking trains around to other countries. We would like to keep the price reasonable though we realize this is not a very cheap trip. So I'm wondering what advice anybody has on where to book flights and hotels, where else to go, and how long to stay there. We're spending the first 3 days in Disneyland Paris for sure. We're traveling this coming summer after we've graduated. Thanks!
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