I recently started designing and I saw these smooth corners people make. My problem ist that I can't find a way to enable this tool and these corner widgets don't appear on my works.enter image description here
As per the comment, it seems you are using a pre-CC version of Illustrator so you do not have access to the "Corner Widgets" which are typically enabled from the View menu. I do not have much pre-CC experience but there are other ways to round corners.
You can make round corners by using the Transform Panel (Window> Transform). You can adjust all corners at the same time or you can adjust one at a time by clicking the chain icon and then entering a radius in each different corner.
Also, you can go to the top menu Effect> Stylize> Round Corners and enter a value there for any selected corner. The exact same "Effect" can be done from the Appearance Panel (Window> Appearance). Just make sure your object is selected and click on the fx icon at the bottom of the panel. Then choose Stylize> Round Corners.These both add an Appearance Attribute which can be accessed from the Appearance Panel and modified or removed later (prior to "expanding" the object).
If you are using the latest CC version then you will get that by using the Direct selection tool or clicking A. But with older version CS6 it is not available, either you have to do the transform all four corners mentioned in the above answer, or just select the Rounded Rectangle tool inside the rectangle tool in Tools panel.