One of the easiest "beginner" terrariums to build is a "butter dish" terrarium.
I bought a glass butter dish from Home Goods for $3.99 and planted it with moss and added some decorative details like seashell and small figurine.
Take a look at the photos. You can bet this will be a popular Christmas gift from me this year!
If you want step by step instructions and materials list, I have it on my blog, EdenMakers.