I have trouble singing in falsetto?

I decided to try out as a tenor for a quartet, and I have to sing super high notes for it. My voice has dropped, and currently I am having trouble with the falstto. I start to sing it, then it falters and I can't seem to hold anything with it..... Any help with practicing or mainting it? because it takes me 3-4 seconds to hit a pitch and it is screwing our practices up........ Thanks in advance!