Welcome. Here are some random thoughts base don personl experience and years of studying dioramas as a judge.
The first step is to plan and plan some more. Sketch things our. Try a layout and take some digital photos and study them to help with better positioning of stuff.
Study some archival photographs for layout and composition ideas. Also consider how the photographer and editor cropped and chose the picture. Guaranteed, for every photograph you see, there are dozens of negatives and proofs in the trash bin.
Try to find Shep Paine's book on diorama building. Use what you see online as inspiration not reference. Pictures of models are pictures of someone's interpretation of somethng else, and very often, that's another model rather than the real thing. Hence we get faddy things like over chipping, heavy to the point of absurd pre-shading, and mud on everything.
Keep your early projects simple, don't be tempted to do Omaha Beach for your first project, rather consider a German machine gun nest on the bluff, for example. Too big a project will likely result in frustration.
Two things to learn early on are repositioning/converting figures, since given poses are rarely compatible with your ideas, and make sure your uniforms and equipment match.
If you're going to use figures from different manufacturers, keep the personal gear and weapons from one source. People may vary in size, but the M1 Garand only came in one size.
Always use some sort of earthwork, such as Celluclay or plaster of pari or other material to give your base a natural feel. Sand/turf glued to a flat piece of wood doess not look natural unless your actionis on a football field.
If you want feedback and direction, especialy in your first atempts, post interim photos here and at other sites so you can make corrections as you go along. generally, critiques here are pretty mild, while at Missing-Lynks they are quite demanding. Armorama falls somewhere in the middle.
Remember, foremost a diorama is telling some sort of story, be it big or small. Make sure your story is internally consistent. If you have an MG team in firing position, there shouldn't be a figure in the scene standing around with his hands in his pockets.