The phrase "first installment" means the initial part or payment of something that will be given or received in a series of payments or parts. Full definition
First installments of the payment plan options are due on join or renewal date. (iaap-hq.org)
This will be the very first installment in the series that'll feature interactive cut - scenes within gameplay, expect an experience similar to Mortal Kombat X story mode. (segmentnext.com)
We answer as many as we can without going too long, making for, I think, a great first installment. (incontention.com)