How can I desinstall anything from Mac I have downloaded here?

The best way to de-install an application is to run the de-installer that came with it. If no de-installer was included, then you can go to the Applications folder, find the program, drag it to the trash, and empty the trash. However, while that's all you need to do for some applications, with others that may not remove additional support files/drivers that may have been installed -- in those cases I'd consult the documentation for the application for instructions on how to remove all components.

Where is an applications folder and where can I find de-installer that came with the app? Right click?

The Applications folder is at the root level of the internal hard disk. If an application installer has included a de-installer it might be in that application's folder or it might be in a support folder somewhere -- you'll have to consult the documentation for each application.

I found that Applications folder is in Finder. There is a list of apps, right click and choice Move to bin. If I move it to the bin and delete from bin, too, then I have desinstalled the app I don't need anymore, right? So I don't need de-installer at all?

Some applications are self-contained and removing the application itself is all you need to do, but other applications install additional items in other folders [Application Support, Preferences, etc] -- usually it doesn't really matter if those items remain after you delete the application itself, but if you want to remove everything then you should either use the de-installer [if provided] or check the application's documentation for instructions on how to remove all installed components.