Depends on what you're modelling.
A structure can be made of cardboard, plastic (styrene) sheets ((visit you local hobby store or find online)), or thin plywood.
You can draw templates on your computer in Adobe Illustrator or CorelDraw.
If you are making a model of an environment, build the structure from styrofoam (be careful cutting and shaping it as it makes a lot of waste which will get everywhere - have a vaccuum cleaner standing by) and then cover that with plaster (spackle).
You can paint it with water based paints.
Plants can be made from twigs, moss and lichen from the garden - zap these in a microwave (old!) to kill any bugs.
Glue the plants with PVA or white glue.