I have been building terrain for 40 years. You will need to start gradually and build up a collection.
Basic advice.
Start with a table cover. Everything else will look better on nice ground cover. Start simple. Go to the fabric store and purchase a yard of felt. Green or tan/brown. Get two colors. The contrasting color will be used to make area terrain markers. Buy some fabric glue.
Stop by a dollar store and pick up a bag of polished stones and a bag of Reindeer moss.Dollar Tree also has some other types of moss that might be useful. You can use the Spanish moss for everything from trees to doll hair.
In the old days people would suggest that you place books under the table cover for hills. Use cardboard cut into shape to start.
The reindeer moss and polished stone will be used with the felt area markers for difficult terrain and will form the basic material for more themed projects later.
Some links
Trees
https://www.youtube.com/watch?v=r5jPBzbpP4Uhttps://www.youtube.com/watch?v=ClCwvzT2VG4https://www.youtube.com/watch?v=mDTKZegWuf4https://www.youtube.com/watch?v=nO4Y1r98X6gHills/foam
https://www.youtube.com/watch?v=qM0vLhZB5QMJoel Logan likes to use cat litter for flocking. I do not recommend it because it generally made from clay. It will not hold up.You can also use a bread knife to cut foam.
Card board crafts
DM Scotty has 300+ this is #1 -Tools start at about 6 minutes.
https://www.youtube.com/watch?v=B4a4m_F ... 2114248595 There are tons more.
Some of my trees
Hirst Arts Project