First of all - welcome aboard - you have found the best site around for modeling.
We'll start kinda at a high overview level and as you starting things just ask more detailed questions. You will find all the answers here!!
First of all
1. Ground works - I use wall spackle from the local Do It Yourself hardware store. To make sand dirt etc. Use sand/dirt from the back yard. You can use Cat litter for larger material such as gravel and rocks. Typically use build a base out of wood, stain it, seal, apply a coating of spackle (or paper mache). Paint it. Then coat it with 50/50 (30/70) water white glue mix. Then sprinkle on the sand/dirt. You can mix the sand with the spackle too if you like.
2. Grass - it can be purchased at hobby stores or you can use hemp rope cut to length or the bristles of an old paint brush. Use water/glue glue mix to fix it down.
https://armorama.kitmaker.net/forums/11708&page=1https://armorama.kitmaker.net/forums/11582&page=13. The buildings are a bit more complex. It really depends on the design.
Here is a good start for scratch building
https://armorama.kitmaker.net/forums/10746&page=1I would recommend reading EVERY DD at this url - Keith is an authority on diorama building.
https://armorama.kitmaker.net/modules.php?op=modload&name=Sections&file=index&req=listarticles&secid=134. Water - its a fun one!!
I personally use the stuff in this article
https://armorama.kitmaker.net//features/150There are lots of other material - stay away from EZwater (The bead stuff), results are poor.
Here is a thread with other products
https://armorama.kitmaker.net/forums/11740&page=1and 5 - The campaigns are 'contests'. There is a defined theme in a sepecific genra of modeling and everyone who wants to join jumps in and builds their thing. Its not a pressure deal at all. You do it to have fun and share ideas and common experiences. Some will have prizes, some wont.