A lot of times it is hard for relative beginners to get good advice. Sometimes, when you start out, there are many things that can be improved on and people are hesitant to reply because they don’t want to seem as though they are being too harsh in their criticism.
It looks like you are starting to get a good grasp on the tools but where you are lacking is on the forms. I’m a beginner myself and am definitely no expert but, in the Batman/Superman pic, pay close attention to the proportions of the characters. For example, both Batman’s and Superman’s necks are really long and Superman’s arms seem short.
In the MiB inspired head, the eyes seem really low and the ears are way too high. But the nose and mouth seem about right.
But definitely keep going. Even in just the few pics you’ve posted, you are rapidly improving.