"I came to throw fire on the earth" or "I came to set the earth on fire." Possible meanings are 1) Jesus has come to judge people or 2) Jesus has come to purify believers or 3) Jesus has come to cause division among people. (See: [[rc://en/ta/man/translate/figs-metaphor]])
This exclamation emphasizes how much he wants this to happen. AT: "I wish very much that it was already lit" or "how I wish that it had already begun" (See: [[rc://en/ta/man/translate/figs-exclamations]])