Amtrak website says:
"In general, northbound Acela trains that originate in Washington have First class at the rear. Southbound Acela trains that originate from Boston have First class at the front."
I'm also taking my first trip on Acela next month. When I look at the seat selection chart, it shows northbound cars 2 & 5 traveling with the highest row number towards the front, and cars 3 & 6 with row 1 towards the front. Car 2 is the Quiet Car.
If you book online, the system will assign a seat, and then you can choose your own seat on the web or app. On the web, first find the reservation, click on Details, and then Modify Trip. Click on the window-pane menu on the right to reveal Change Seats. I expect the workflow is similar on the app.