You need to be licensed. Take a bar exam for a state, and then there's a separate application (not an exam) to be admitted to that federal district. It's often (at least for me) just a question of filling out another form and paying a fee. After three years of admission to any bar you can apply for admission to the supreme court bar.
Even if you want to be a JAG you have to first pass a state bar, any state bar, even though you will only be practicing for the military.
Taking a state bar exam is what virtually every law student does the summer after law school. There would be almost no reason why someone where the situation of someone getting an opportunity to be a federal attorney without taking a bar exam.
Bear in mind that in reality most big-firm lawyers end up "practicing" law for several months before they are admitted to bar, government too. There are also ways to argue in Court without being admitted first (think Legally Blonde). But these all involve direct supervision of a senior attorney who puts their seal of approval and their name on your work.