I know i won't be happy if i get my 2nd choice

Honestly, there's really not much you can do right now. If you get your 2nd choice, go through the process before initiation, and see if you're happy or not. If you're not, then you can drop before initiation and go through FR or the subsequent COB next year (several girls have done this). Initiation is usually a couple of months from now, so you have time to see whether you vibe well with the girls in your chapter. My recommendation is to go in with an open mind - there's a reason why your 2nd choice ended up being your second choice and not your 3rd, 4th, etc, and don't shoot yourself in the foot by saying you won't be happy if you get your 2nd choice, because then you'll be biased the entire time and not have any fun.