Absolutely the most important thing to do is what *shininginthedarkness said, no one can help or give you pointers if they can't see what level you're at or what kind of mistakes you're making. No matter how bad you might think it is, absolutely post you work up and don't delete it either if it starts ageing. It's always good to keep a record of older works to compare your level of improvement. I'm a wannabe manga artist who's been drawing for about 18 years. I'm no professional by any means but I think I could help critique and point you to some resources once I can gauge where you're at. Even though manga isn't what you want to do, it has elements in some of the things you favourited and all art has the same basic foundations anyway. I added you to my watch list but you're welcome to poke me if you want me to critique or give advice on specific works. I watch a lot of groups and deviants (hundreds of watch messages a day) so sometimes I miss people's latest uploads in a flurry of delete message clicks since I can't look at everything.
I recommend you either start posting some things and go from there to improve, or keep practicing until you reach a level where you're comfortable with sharing it. Only once you start submitting art, you have a chance of getting helpful feedback to it. But whether you submit already or not, you can browse tutorials, and most importantly, browse art from other not-very-well-known artists (I recommend setting the front page to 'newest') and give them feedback. That's one of the best ways to learn