How to start?
Fetch a glass container (round or rectangular) with a sealable lid. Start with a layer of gravel, a layer of sand, then charcoal (half of sand) and top it with potting soil. Make sure there are no clumps in soil – mix it very lightly with sand for a well draining texture. For plants, preferably use moss and button ferns, crotons and slow growing plants for the tropical feel. Avoid cacti since they suffocate in excess moisture. Do not mix desert plants with moisture loving tropicals. You could try herbs too, but pruning might need extra attention.
Once you have found your plants, carefully plant a small sample of plant you choose. Make sure you give it some room to breathe and grow and water very lightly, just spray and replace the lid. No direct sunlight is needed. Ambient light will do else you’ll see condensation on the glass. For the next few days, watch your terrarium closely, and see what balance of moisture and air it needs. Depending on the size of the plant it may need more or less water than you’re giving it. If it seems as though there is too much water, leave the lid off for a day, and let it dry out a bit. Make sure you remove dead matter, trim plant on a regular basis. Once the terranium looks stable, water once in few weeks and don’t care. You could also design a landscape within, create uneven surfaces (hill look with lumps of soil covered with moss) and grow plants in sections. Use bridges, craft animals, birds, other structures for a dramatic presentation. Once you are comfortable with the terranium environment bit and feel confident to experiment more, you could move to other plants like mini-foliage and flowering plants.
Just in case you do not have lid for the terranium, you will have to care for it a little more. The open-terranium needs to watered once in few days unlike the closed ones. Everything remains the same.