"Knowing what you need to do" is different from "defining what you are going to do".
When defining what you are going to do, you are considering your constraints, such as resource availability and time.
Resource includes People, Material, Equipment, and Money. Available means when you need them they are there. It is beyond whether you own it or not. You may have assets, but you do not have cash on hand, you do not have Money. If you have resources but they are working on other stuffs, your resources are not available. When you define the human resource requirements, you need to consider skills. People with different skills are different resources.
With the constraints in mind, selecting the items from the list of what need to do and putting them into the list of what you are going to do" is called "Defining the Scope".
In the scrum method, the action is to call a scrum planning meeting.
- The list of "what you need to do" is your "backlog".
- The list of "what you are going to do" is the list of tasks in a sprint.
A good tool should allow you to create such lists and drag and drop back and forth between these two list during planning.
A tool tool should allow you to define the resource requirements and aggregate at the spring level and to show if you are still able to do them without going beyond your constraints.
Once you are OK with the list, you should be able to freeze the scope.