Ok, here goes...
First of all, let me say that in no way do I have anything bad to say about it... At your age, it's a fine effort, just needs some tweaking and a bit more attention to detail... I'll be pointing out some things that will help you improve your dioramas in the future...
Composition and Layout:
Although your layout overall is ok and balanced well, there's no clear focal point for the storyline... The only interaction I see is the Ivan pointing the jep off to another direction, or telling him where to park, it's not clear which... The Sherman is being ignored at the same time, and it appears it's stopped, but there's no open hatches, no idea what he's doing there.. The Yank with the bedroll standing next to it is something of a mystery.. If he's going to toss it up on the tank, why? He's being pretty casual about it too, since there's a Russian about to shoot something right there..
The figures at the right rear behind the wall appear to see something, but what? Also, your figures in general are running around willy-nilly or looking off in all directions...
Groundwork is not very spectacular, not sure exactly what kind of "earth" you got going on there... It looks like carpet to me... The vegetation is way too sparse as well... Tanks are really heavy, and even the lightest of them tear hell outta the ground, escept for hard-packed and paved roads... You need some tracks from the tank, for sure... Another thing about the groundwork that you should always do, is to carry the ground-colors up onto the lower parts of the vehicles and figure's feet... This "ties" them to the groundwork and makes it appear that they belong there.. Also, the vehicles could stand a bit more weathering on the sides..
Now, were it me doing this with what you have on it... First, I'd lose the Soviet rifleman up front and the GIs at the back... You could fill that space near the wall with some foliage instead, a sign in Cyrillic where he now stands... The tanker should be talking to someone up on the tank, and you could put an Ivan there as well... I's also put the other Russian by the jeep talking with the driver (or more likely, the passenger, since it's more likely that he's the one in charge, a company commander (Captain) or Platoon Leader (1st or 2nd Lieutenant), perhaps).
The Base: Remember, a diorama base the "frame" for your picture. It looks like you built this directly on a shelf of some sort. A stand-alone base is better, one with either a nice wood veneer on the sides, or sanded and varnished/painted edges... OR: If it is part of the shelving, you're halfway to a shadow-box diorama here... Just add a front panel with a reveal, a background, and add some interior lighting and you're on your way to the next level of diorama-building... But that's another story.. You can start with what I've said, and take it from there.. Others will chime in with observations too... Everyone here will help you out as much as we can..
At any rate, you're doing ok...Just keep plugging away at it... Keep that one as a bench-mark for your future builds too.. You'll always be able to see how far you've come with each successive build...