ill would like to take the JN0-102 exam soon.
how can i ger a voucher for this test??
there are generally no vouchers for the JNCIA certification. If you are a partner/disti SE there are some ways to get a voucher or discount on JNCIA if you do some online training before hand.
Official certification tests are booked and payed directly to Pearson Vue: http://www.pearsonvue.com/junipernetworks/
A JNCIA test will you you $200 USD.
which tests should i take to get voucher?
I have just looked at it; Your company needs to be an official partner and you need to be registered as an engineer in the Juniper Partner program. Then do online traning to become an "aspiring champion" to get the voucher.