Welcome to this forum and all the best to your bright future in designing work. There are many platforms where you can showcase your designing work. Like Instagram, Facebook, Behance etc.
Hi! welcome to this forum, you can upload your work and ask for a critic. There are many professionals over here who would rate and comment on your work and would also give you some suggestion to make your work better. You can find the critic forum and post your work over there.
The designs you choose to work on and showcase can be anything here in this site (make sure the topic first). It's understandable if you do feel qualified to judge people's comment for your designs yet. Welcome to the forum!