Read the different flyers at this link to get a good starter course in dioramas from the original master. It's old, but the information is as true today at it was then.

http://sheperdpaine.atspace.com/

This guy and these flyers in the kits were an inspiration for so many of us.

You can use anything you like as a base, i use old bits of furniture chipboard. Just depends on what size you want it. I don't most of those other things, so can't help. but theres a wide range of products that can be used to make a base. For the foam i use what ever comes in parcels used as packaging. I've never needed to buy any.

Bish: "chipboard". Translation from English to American: "Masonite".

There are two basic kinds of styrofoam- bead and extruded. Stay away from the beaded, it's miserable to work with except as maybe the base sheet over which you glue higher quality stuff. Keep an eye out for the extruded pink or blue stuff in builder rubbish boxes and when you see a piece, grab it.

Plaster is ok except it is heavy and will crack if you are inclined to move your dio around. Joint taping compound for drywall works well and it comes in a tub, so no mixing.

Shep's book and articles are all about composition and detail, classic stuff.

My only other advice for a beginner is to keep it small and straightforward. Pick one central object as the focus. And avoid dead people and gore unless it really matters.

It's fine to have one or the other side engaged in actual battle, but when you add the other side to the dio scale becomes a problem. Even at 1/72 scale people shooting at each othe need a couple of feet apart to be believable except in an ambush situation.

Any time I build a diorama that's anything more than the base for a model, I usually make a model of the dio, say 1/4 the size of the final one, out of scrap styrofoam, bits of wood, paper. That way I get a skecth of the design to study and modify before I start slapping spackle around.

Have fun!

You can do this first; go to the thread titled Dioramas; You did that, so you are definitely on track!

 These folks can give you the best help possible. You can do Like I do.I start with a piece of Wave textured Glass, In either rough or smooth light wavy surfaces,  in the closest color I can that has enough clarity to paint darker under it. Mostly it's a piece of Plywood or scrap Plexi.

Then a discarded picture frame the right size or just a piece of nice wood. Then I hit Hobby Lobby for resin or railroad base if I need it along with the resin. Other than that you need to pick the brains of the Dio-Masters on the thread!

Yeah, I can't recommend Shep Paine's stuff enough, a lot of the detail stuff is dated but the overall techniques are timeless. 

I'd go with Durham's Water Putty over plaster. They claim it dries rock hard and they ain't kidding, do any shaping of the putty before it dries, even trying to shape it with a Dremel tool is a pain afterward. But it won't chip or break like putty, you have to pretty much hit it with a hammer to shatter it. You should be able to find it at most hardware stores. 

You don't actually want thick, heavy plywood for the base.

1/4" plywood, masonite (it's a dark brown color and smooth on one side, typically) will suffice.  All you need is some rigidity.  Especially with foam used as a base.

Yes, the blue (good) or the pink (better) foam sheets at the home center are good stuff.  Fine yellow foam is likely to be Isocyanurate, which is really good for roofing, but lousy for dioramas (it does not cut with a hot wire for one).

You might want to invest in a wire cutter from Foam Factory.

Many things at the home center are useful.  Grout mix is a very stable "dirt" mixture.  Premixed drywall compound and patch material are handy (as is cellutex, a paper-mache product [in Eurpoe sold as Sculptamold] which is more an art store product).

While in the home center, they also stock rolled wood veneer (or ought to).  This is a handy thing to apply to finished edges of dioramas.  You can get moulding shapes used for door & window trim as well.

Let me recommend Luke Towen and Kathy Millet on YouTube for having ecellent how-to videos for diorama videos.  Another good video creator is Laser Creation World--but that's a very German site in terms of materials used (they market a bunch of supplies, themselves, too--which can cause tool envy of great proportion).

Tomas at LCW has a weathering technique that is fine-art based, rather than more modeler-based, and is fascinating to watch.

Second Luke Towan's Boulder Creek Railroad website and his video tutorials.   Here is one showing how he made a small diorama of a small stream next to a bluff.   He uses expanded polystyrene foam for most of his structure

https://youtu.be/MGKmhmo79zw

He often builds on a MDF base with picture framed edges.   Groundwork is often sculptamold fibrous plaster.  Follow him as he shows how to apply groundcover, grass, shrubs, trees, and more.   I like how he does his still water, his waterfalls push things, and his ocean is too much.  He has downloads of some of his 3D print files.   He does solicit support through Patreon, which gets some other access and hoohahs

stikpusher

Read the different flyers at this link to get a good starter course in dioramas from the original master. It's old, but the information is as true today at it was then.

http://sheperdpaine.atspace.com/ 

This guy and these flyers in the kits were an inspiration for so many of us. 

I second that, Stik!  I recommend Shep's book, too, "How to Build Dioramas".  It goes into more depth than the "Tips" brochures.  His tips are as valid today as they were when he wrote them, and while he used some off-the-shelf products, like some model railroad foliage, he did a lot of scratchbuilding and kit-bashing.  For example, Shep used  some HO scale balustrade, a rear seat from a 1/25 car model, and some other odds and ends to make a settee in one of his 1/32 dioramas.

His goal with the book was to show the average modeler, and the beginner, what was possible, starting with your own imagination.

Don't let the images you see online be your yardstick.  A lot of the content is made by modelers with years of experience, some of whom are professionals.  You'll get discouraged if you expect your work to be as good as theirs, right out of the gate.  Keep that in mind.

As to your specific question about the base, yes, you can use plywood.  Whatever wood product you use, it's a good idea to seal it before you start adding groundwork, especially if the material you use for groundwork contains water.

I use pink insulating foam for ground contours, which I fix to a wooden base with white glue, and I like using wood putty from basic texturing and to build up contours over the core.

One idea for a base is a picture frame, glass, wood or cardboard inside. You can build your base in the frame and hang it on the wall if you want. Of course it depends on the size and weight.