だって、[sentence] [noun] + だって [question word] + だって [noun] / [na-adj] + だって。 [verb (plain)] / [i-adj] + んだって。
だって is casual speech, and its meaning depends on where it stands. At the start of a sentence it brings in an excuse or a protest, "but…" or "because…", and the reason that follows often ends in もん or んだ.
This だって answers a question or a complaint, and sounds defensive, even childish. In polite speech, give the reason with きごう〜ので or きごう〜からです instead.
After a noun, だって is a casual でも or も: "even X" or "X too".
After a question word it means "any… at all": だれだって誰だって is anyone, いつだって any time, なん何だって anything.
At the end of a sentence, だって passes on what someone said: "I heard", "they say". It goes straight after a noun or a な-adjective; a verb or an い-adjective takes んだって, or just って.