not bad, but not that good, either.
it's pretty clear you know what you're doing right now - the movie has, what, 5 songs or so in it? and it's less than one megabyte? excellent job on the compression, especially considering it's also a sprite movie. all you need to do, in order to get better, is just practice. instead of using pre-made art, try drawing things yourself (the tank was a nice, if not out-of-place, touch). the animation was pretty good, but try increasing the framerate a bit and make movements a bit smoother. the tank's movement was probably the worst, and the explosions were pretty lackluster. instead of a fireball sprite, find an actual explosion sprite, and don't just tween it around when there are more explosions. the dialog wasn't that great, but at least you had decent grammar. many first-timers tend to be young, and as such, don't care about grammar. so that's a big plus for you. just make the fonts bigger and easier to read. also, for a more cartoony look, give the speech bubbles a thicker outline of 3-5 px. thinner ones are used for more realistic sort of cartoons.
good luck with your future flash endeavors. remember to not just stick to spriting, though (: