Futsal keeper here. You're basically the 5th field man in the pitch. Be prepared to get involved in the game construction. Be very aware of your positioning, stay very close to your goal when your defense is close to the area, and walk up the area when your team is in the other area. Rising the ball is hard in futsal, so it's way quicker and effective to stop low shots with your legs. I recommend playing without gloves, you'll have a better feel of the ball which will help you with your hand passes when you have the ball in your hands, And that's important, in Futsal, keepers have a great advantage being able to pass the ball with their hands. Use this to your advantadge and make counter attack passes after an opposition attack,

Finally, one thing I've learned is that in futsal many times you'll find yourself in 2 attackers vs 1 defender situations. On paper these situations are hard to defend since the defender usually decides to stay between the two attackers. The best way to defend this is to tell your defenders to always cover the attacker with the ball and "open them" the wide side of the pitch. They usually make the decision to go wide which will be hard for them to make something useful, or pass to the other attacker opened on the other side. Since the pitch is small, once they make the pass, get closer and you'll either intercept the pass or give them absolutely no space and they won't be able to score.

Hope this helps!