You've gotten a lot of good advice already, so I'll just agree a lot and drop a few tips.
Don't be afraid to copy. When I was first showing my work at conventions, that's the biggest advice I got was to look at how other artists handled things and then do that. If you're drawing Spidey, look to your favorite Spidey artists and start copying poses. You'll learn how Spidey moves and how everything connects. You should also see the importance of line variation, but I'd focus more on your figure work at this point.
Use toys. You're drawings will look a little stiff, but if you've got a highly poseable action figure, you should be able to work out a specific shot with that character. I like to use some of my Marvel Legends toys when setting things up. You get an idea of what the lighting on the character would look like and you've got a three dimensional model right in front of you, so your art should look more 3D. Don't use it as a crutch, no toy will offer you the same range of motion that a real person has (which is why it'll look kinda stiff in comparison), but it's a great tool for proportion and just learning the basic anatomy stuff.
Draw things over and over and over and over. I think I'm pretty good at drawing faces, but the only reason for that is when ever I had an empty piece of paper and some time, I'd start drawing eyes and noses and mouths. Sometimes all together, sometimes just a page of eyes or mouths. Some of the most difficult things to draw are faces, feet, and hands. If you want to be good at 'em, you've gotta keep drawing them and sometimes focus on JUST drawing them. I'm currently working on hands. Feet? You can hide them behind a bush or convenient mist.
Anyway, you've got some nice talent that you just need to keep working on. You'll see improvement with almost every drawing. Once you get some more of the basics down, you'll be able to branch out and develop your style in any way you choose.