Why/Because なぜ、どうして、なぜなら

2 ways to say "why" are:

1. なぜ naze - why
2. どうして doushite - why

They are basically interchangeable and start at the beginning of the sentence and are followed by the question

なぜ（どうして）　私　の　ケーキ　を　たべました　か？
naze (doushite) watashi no ke-ki o tabemashita ka?
Why did you eat my cake?
[There isn't a "you" but obviously you wouldn't be asking yourself this question.]

なぜなら + reason or excuse + kara

なぜなら　はら が へった　から。
nazenara hara ga hetta kara.
Because, (I'm) starving! 
[lit. because stomach is diminished]