Make sure you have a desire to want to report on Congress. Get up to speed on current issues facing Congress, particularly if you know the issue area for the position you are applying for. While CQ is behind a paywall and you might not have access, read as much as the public Roll Call site as you...
When Congress is in session, you are at the Capitol with them: meeting with staff, covering hearings, staking out members in the hallways as they meet privately and vote, asking members questions and putting everything you've gathered together for a story. You don't necessarily write a story ever...