I didn't decide to be christened, and I wasn't aware of being christened at the time. But a lot of us who would call themselves Christian do so because that is what they have been culturally brought up as. So to call oneself a Christian in this case is to accept your upbringing, even though you didn't get to choose your upbringing. So in a sense I was happy to accept having christened even though I was not aware at the time.
I did worry about what to say, the expectation is you will say something when you get baptised which is called giving your testimony. The ideal testimony follows the apostle Paul and his Damascus Road experience. The true saints are able to tell you down to the second when they made the decision to give their lives to Jesus. I am a gradualist in many things, and for me faith is a process not a one-off operation. I'm not alone in this.
I ran over various ideas beforehand. Almost on the spur of the moment I found words to say that I was happy to say yet satisfied those listening. I was bemused to hear people saying they appreciated what I had to say as I thought it was rather fuzzy and woolly.
Getting baptised itself was unusual. Being pushed backwards under water is not something I'm used to, or was looking forward to. For some people it is a spiritual moment but for me the mechanics of the baptism dominated my thoughts. Would I resist being pushed under water? Would my feet go up in the air? Would I remember to not wear my watch and phone?
Did it change me myself? No, but it was something I needed to do for completeness, for harmony.