Typically, opening nights are invited only. Though sometimes, a block of tickets will be released to the general public.....
If you want to attend an opening night, you can always try going to the box office (you'll get better results than telecharge or ticketmaster) a day or two before to see if they have anything available.
Other than that, unless you're in the business, or friends with someone associated with the show, an opening night is rarely something you can plan ahead for.
"You're every gay man's wet dream!" ~ MA
If in Heaven you don't excel, you can always party down in hell...