• Ask your teammate what he studied to get better
• Watch a lot of good debaters; note what they do that is good
• Watch some bad debaters; note why they were bad
• Debate, debate, debate, even if you have to do it over the phone

You don’t. Practice makes perfect. If you want to get good at something, you have to put in the work.

Actual practice is the best answer, as everyone has already said, but to tag in something you can work on outside of real debates -- work on controlling your emotions so they don't interfere with your mind.

How do you get better at playing the guitar without actually playing the guitar?

(edit): Point taken, but “playing the guitar” still wins by a mile

Learn to spot logical fallacies and how to exploit them. And never let them put you on a defensive to the point where you lose focus of your point.

learn to be skeptical, recognize logical fallacies, and never get angry.

Studying philosophy can help. Try writing your take on what you’ve studied. Offer it for critique and try to understand any flaws that were pointed out.

Nice try, Donald.

Debate.

You don't need to do this in a formal setting, I think groups like Philosophy clubs are excellent for learning how better to support an argument (and they're usually quite positive and supportive settings).

Imo the only way to get better at debating is exercise. If you have an idea of the topic it's always a good idea to do research to build your familiarity but rhetorical skills gain with practice.

Don't rely too much on ChatGPT here. It's not a human and isn't going to respond the same way a human would.

If you over practice with that tool you might become great at debating ChatGPT but that may entrench bad habits that hurt you when debating a human.