I think one thing that helps is studying more yourself, and only speaking on subjects you're confident in your own understanding. For example:
if communism is so great why did it fail everywhere
By what metrics? Socialist China is rising rapidly, and socialist countries do better than peer countries when it comes to quality of life metrics like life expectancy, despite increased international strain. I also recommend reading about some of the problems with the USSR and how it was dissolved, so you can prove that it wasn't simply because they were socialist.
Communist were just authoritarian
Ask them to define this! What tends to help is highlighting how the communists, when in power, asserted the authority of the working classes. This is in stark contrast to capitalist systems, where capitalists assert authority.
China is just as bad as the US
Again, how? China has much higher approval rates, engages in win-win economic cooperation instead of naked imperialism, only has a couple overseas millitary bases while the US Empire has hundreds, and maintains a defensive millitary instead of offensive. Plenty to show!
People often focus on argument before becoming confident in study. The rest, like learning how to engage and reach people, comes with practice and time.