It seems you've run into a bit of a programming error. Not sure why there's a letterbox and why that letterbox doesn't properly serve as a border. I suggest adding a dynamic camera (or vcam as some people like to call it.)
But that aside, overall for your first submission there's nothing particularly bad here, your humor is reminiscent of classic Looney Tunes-esque cartoons, which is always welcome. But you could have done more. Perhaps animated the tree a bit- have it shake or bend slightly when it's hit- or at least provide a sound effect to make it clear that it was in fact hit and not just a piece of clipart on the screen.
Adding sound in general would help this as a whole. But overall, this is a good effort- especially for a first upload here, and ESPECIALLY considering many people just start off with submitting barely watchable one on one stick figure fights.
I look forward to seeing more from you.