A few minutes ago after the World Cup ended, Shinji, guitarist of SID, tweeted this:
胸が熱くなりました…(^-^)/日本に勇気と希望をありがとう！
I'm having trouble understanding 日本に勇気と希望をありがとう！
Does it mean, "Thank you for giving courage and hope to Japan," or does it mean "Thank you Japan for giving me hope and courage?"
Sentences with に in them are often confusing to me.
It is the former. The full sentence will be: 日本に勇気と希望を(くれて)ありがとう. 有難う is literally 'it is difficult to happen' or 'it is rare, and hence a valuable thing'. There is no room for a ...に phrase or ...を phrase to appear within the frame of the predicate; you cannot attribute the rareness to someone.
This kind of phrase is often heard right after Japan had went under a disaster. Among the famous one is, when Japan was about to recover from the disaster of World War II, Yukawa Hideki was given the first Novel prize in Japan, and similar phrases were seen all over in Japan. The text you cited is probably based on the recent earthquake and tsunami diaster that hit Japan.