Presuming you are using Board layout, there are two buttons to add a new task. Click on + icon on the top of the column and it will Add task on the top of the list.
If you use the “+ Add task” button at the bottom you will add task at the bottom of the column.
If you are using List layout:
Select the task below which you want to add a new task and click on “+Add task” button and it will add the new task right below the task you selected
however if you select the “Add task…” at the bottom of the list, the new task will get added at the bottom of that section.
Hope this helps!