Those look like some type of java fern to me so first thing first - dig them out from your substrate. The rhizomes need to be exposed for java ferns to survive which might explain why they're dying. You'll usually see them attached to rocks or driftwood. You can
bury them but make sure the rhizomes are exposed at the very least. The other thing looks like moss which is fine as is or you can tie the java moss to a rock or driftwood so that it just isn't a floating ball :P
The good news is those are low-light plants and don't really need a lot in terms of maintenance and care. They will grow as soon as you pay a little attention to them first! Honestly, I would just do a 50% water change (and fill the tank up, you have so much room at the top), re-plant the java fern properly, maybe do something with the moss, and let it sit for a few days to a week and see how it goes. As for the guppies, I'm not sure. I only deal with shrimp for now.
In terms of decor, I bought my river rocks from Petsmart for $1 a bag. I got like 5 really nice rocks from it. I still might have a manzanita branch or two I can send you if I find them ... they're ... somewhere ...
You have great stuff for a low-tech start with low-light plants. You just need to put a little love into it, that's all!