There's not a one click solution for making things realistic. You have to match the light setup from the original scene, so look where the sun came from etc. But reading your post, I get the impression you don't have a whole lot experience with it yet, since the way to make a shadow like that in Element 3D is by using the matte shadow material.
It's much easier to talk about this when you show us a screenshot, like this it's just guessing what's wrong with it.
Greetings from the same country.
There's a reason why big-budget film and TV may have multiple artists, sometimes dozens, working on just one shot for weeks on end. Compositing is a very complex and skilled art form, that takes years of training and development to be the best. That said, many tools like Element 3D bring incredible power into the hands of us all, so don't be disheartened.
If you can post a link to your work I'm sure you'll get lots of suggestions. Typically, the big issues that make basic compositing look good and bad are things like colour grading (colour temp, black levels etc), atmospherics (fog, dust, light spill, volumetric light) and the list goes on.
Like how do you make your CGI look realistic?
Years of experience. That's all there is to say. You have to understand how lighting and surfaces interact and how specific shaders in 3D tools need to be tweaked to capture those subtle things. You don't learn this just by following tutorials. It'S a continuous process of hands on work and every render gets better. That and of course the quality of the model and level of detail play an important part. If you post an image, we may be able to advise for this specific scenario, but unless you realyl commit to learning this stuff, you will run into the same problem the next time. That's just how it is.
And never forget to just look outside and just see how light works in real life.
Of course like said before, on big budget productions there's a whole team working, but with some productions a decent result will be enough as long as it doesn't distract.