It is entirely up to you...
Reasons to have the surgery first might include: financing, insurance coverage, unbearable chest dysphoria, access to preferred surgeon, scheduling concerns (work, school, etc), unsafe or uncomfortable binding, or just that you WANT it now.
Reasons to start T before having surgery might include: financing, possible shrinking/loss of breast tissue on T, anticipated weight loss or gain, desire to build more muscle before surgery, scheduling concerns, access to preferred surgeon, uncertainty about wanting or needing top surgery.
And you may have other factors that influence your decision...But it is just that, YOUR decision.
Personally, I had top surgery about 3 months before starting T. I was fairly large chested and extremely BOTHERED by that! They just needed to go... I have no regrets.