Not bad for a first. I can see why you're not so sure about it, though.
For one, there's no environment. There is light shading off to the right of the characters implying a light source over our left shoulder, but the way the shading is dispersed makes it seem like these characters are on a white sound stage. Y'know, those sets with the curved transition from floor to wall? So these characters are staged atop and in front of nothing. It would behoove you to practice with even minimalistic environmental elements to imply size. Are these children, elbow height with an average bed? Are they tall? Are they kaiju sized? Maybe they're like Galactus and these celestial titans are battling on the edge of existence itself? We need context.
Another weak point could be anatomical decisions. There's not a whole lot to go on with the wizard, because with their back turned, concealed with a cloak (as wizards are won't to do) not much needed to be done with him. He was a simple addition to a scene with a minimal amount of appendages protruding out from the cloak...if you were to challenge yourself, you could add dimension to the fabric of the cloak; wrinkles, tatters, weathering, layering as if it wasn't just one piece of cloth bought from a Halloween store.
But from what we can see of the wizard, the boots seem to be pointed in the right direction, and his thin arm is definitely clutching his staff...Staff of Lemons? Is that a Lemon wedge at the top of the staff? Perhaps an Orange? Is this the Citrus Wizard? (I'm kidding.)
Now, it makes sense that the wizard's arm is thin. Wizards don't rely on brute strength, they're cerebral, other-worldly, and what makes them powerful is magic. A knight, on the other hand, has to travel through battle weighed down by heavy armor...swinging heavy weaponry hard enough to cleave armor, flesh, and bone...The decision to make HIS arms as slender as they are were a creative mistake I think.
It makes sense that his chest is broad. But if you look at the distance between his shoulders, and then you look at the thickness of his arms, it doesn't seem to go with one another or make sense. Especially as the forearm/bicep transitions into that unnatural looking elbow fold.
The most valuable research and practice an artist will EVER invest their time into is anatomy. Anatomy of each sex, anatomy of each body type, its a CRUCIAL skill to know. Do you want to be Adventure Time or do you want to be Frank Frazetta? Stylization is perfectly fine, you just need to know what you want to become.
Having said that, his face and legs look great in composition, but his arms throw it off. The sword looks great, but its distracting that his hand is clutching the dead middle of the grip, and not closer up to the hilt where he would have a sturdier hold. The shield looks fine, but with no detail added to it, it seems like a prop to conceal and save you from drawing a third hand in this shot. I applaud you for drawing all feet, however.
Make no mistake, this is a great first submission. Keep practicing, and keep challenging yourself by stepping outside of your comfort zone.
-1 star for scanned in pencil sketch
-.5 of a star for lack of environments
-.5 of a star for anatomical decisions