The 'break-even' line in Arena 3 wins. I have been saving gold for Naxx and have been away from Arena but iirc, you get your entire Arena investment back at 3 wins. Everything above that is profit.
I do agree with reading a few articles, watching a few arena runs on youtube, and then doing some Arena runs as the best way to get into the game. Don't try to compete on the ladder right away because you just can't. Your best Legendaries are (debatable and in no real order) Ragnaros, Bloodmage Thalnos, Ysera, Leeroy jenkins, Grommash Hellscream, Tirion Fordring, Cenarius, Al' Akir, Harrison Jones, Nat Pagle, Sylvanas, Cairne Bloodhoof, The Black Knight, and I like Malygos. (I'm sure I forgot one.)
Play often and have fun, that's the most important part.