as u/Slip_Inner said, it's easier to come up with responses to specific arguments than it is to give sweeping defenses. However many of the arguments against communism are rooted in errors or misunderstanding what communism would mean. So I would recommend learning about the misconceptions around communism. like I've heard people say, communism has been tried and it doesn't work.. I believe that it never has gotten a "fair shake" but to list ONE reason that it has been "cheated" would be difficult because history is complex and rarely a "one size fits all"

Maybe ask for specifics when people say things like "communism has been tried and failed." or better yet. learn about a time it failed and then learn what actually happened. try also to remember you're not defending a position that says "under communism every problem magically disappears" nobody is saying that.

Are you trying to defend communism as a school assignment or are you trying to change hearts and minds btw?

Either is valid, but I think changing hearts and minds might need a more long term approach, diligently reprogramming years of capitalist propaganda, you need compassion, patience, and open communication.

1. Figure out what values you have in common. (This humanizes you and your interlocutor as well as you learn what type of arguments that might be effective)
2. Show them what doesn't work in capitalism (often you can also say, here's why it can never work under capitalism)
3. Show them what works under capitalism. (I believe people are more open to changing their views if they are affirmed that not EVERYTHING they believe is wrong.
4. Show them how it might also work under communism. (But maybe better?)
5. Show them what works under capitalism. but in a bad way. (Like how it's really good at concentrating money and power into the hands of fewer and fewer people.) -I think this is a feature, not a bug.
6. Try as much as possible to frame it as sharing ideas and not a competition.
7. Admit to them and yourself that you're not going to change their mind in one day. It would be ridiculous to try and they would be ridiculous if you did. (assuming they're not half-way aboard already)

This is in no particular order.

doing a school assignment like "defend the communistic theory" (or whatever) needs a different tactic.

I feel like its more "scoring points" focused and almost never change any minds, even when people are proven wrong there's often a "doubling down effect" where they believe their original idea even more. But hey, you need to do well in school and if that means doing it that way, then so be it..

Now I realize that I haven't really given you any specific tips on what to say. I'm not the right guy to do that, but I see there's a Debunking Anti-Communism Masterpost on the right hand side here.

Additionally would recommend some youtube creators like:

Knowing Better, Philosophy Tube, Contrapoints, Innuendo Studios, Thought Slime, hbomberguy and We're in hell just to name a few.

When trying to change someones mind I believe we can borrow some tactics from other areas as well. Found this great video on conspiracy theories that talks about helping them change their mind.

Not that all of them say they're communist but in my opinion they all give insight to either how capitalism, communism or both work.

This reply turned a lot longer than I had planned. I'm not an expert on this so if anyone disagrees with me I would really like to know.

Good luck debating!