Prioritizing project task

First of all, task must not be prioritized if you are talking about scheduling. The task duration and relation with other tasks will give you the priority. Second, "everything is urgent" is a fallacy by definition of urgent. You have to work on the definition of urgent or the perception of urgent inside your organization. Including yourselves. There is no other way to deal with that.

Project Schedule Management Knowledge area as per PMBOK Guide 5th Edition
has the following discrete steps

Plan Schedule Management
Define Activities
Sequence Activities
Estimate Activity Resources
Estimate Activity Duration
Develop Schedule

Sequence Activities is a key step because it helps you build the Schedule Network Diagram. The diagram will tell you which activity will be the predecessor and which one would be the successor . It will also help you determine alternate paths to achieve the project objectives and find you the critical path of your project, which is the shortest path (Duration) from the start to the end of the project.

Not to mention , also helps you determine Finish to Start (FS), Start to Start (SS), Finish to Finish (FF) and Start to Finish (SF) dependencies. MS Project will also let you define these dependencies.

You can control the schedule through Resource smoothing and Leveling activities.
once you have done this on paper , you can leverage MS Project to help you build your milestones , activities, overall schedule etc.

Also useful is the concept of Rolling Wave Planning. If you are able to break down a work Package, that you know , needs to be executed in the very near future, you can break that work down into detailed activities and leave the later work package at a higher level. As you get deeper into the planning phase, you will identify more details around a work package and can then elaborate it further.

Prioritization of task require complete knowledge, details , size, complexity of project and priority of stakeholders will define the prioritization of tasks in projects.

When I have multiple activities, I usually give priority to those activities that are keeping someone else from proceeding with their activity.

Beyond that, it really boils down to assigned/negotiated target dates.