How to free up RAM on a Mac?

There are a wide variety of causes of "slowness'. If the reason in your case is not enough RAM, then you need to add RAM. If it's because of other reasons, you have to address those.

Are you talking about disk/storage memory? or RAM?

They are two distinct entities. Deleting files off disk won't (typically) affect the RAM usage of the machine.

RAM is used by running applications, so the easiest way to free up RAM is to quit applications that you're not using.

unistall useless apps and clean junk files

Both these steps are admirable, yet irrelevant as far as RAM is concerned.

Next is to look at what applications are running, how much RAM they're using, and which applications need to be running.

Fortunately, Activity Monitor makes this easy for you - open /Application/Utilities/Activity Monitor.app and look at the Memory tab to see which applications are using the most memory. Decide if they need to be running.

It's also possible that the problem isn't memory, per se., but CPU (one rampant process can degrade performance across the entire system), so switch to the CPU tab in Activity Monitor and look for any abnormal usage - most apps should be using 0 - 1% of CPU.

Do any of these steps help?