Many folks are daunted by this task, it's not rocket science, ya, you might kill some plants before you get it right. It's ok.
The first thing to note is drainage, and how there is none. Some folks put a layer of activated charcoal on the bottom which helps to prevent fungal growth. A layer of small rocks can create a faux drainage layer. While both of these things can be helpful they are not necessary for success. A nice feature of clear glass terrariums is that you can clearly observe the moisture level and water as needed. Don't water if you see that it is adequately soaked, I often let mine get dry before I water. Overwatering will lead to rot and your plant will go to mush.
Light. Generally you want to avoid too much direct sunlight, bright indirect is best. Succulents are a popular plant for terrariums, but they can be tricky! The most common "unhappy" sign I see is when a succulent, which should be a tight compact flower or cluster, begins to elongate and stretch out, creating lots of space in-between each "leaf". They are reaching for sun. Find a brighter spot or consider replanting.
Get creative! Decorative rocks and sand make a very nice finishing touch to a terrarium, I usually buy aquarium rocks from my local pet shop because they have the really small pebbles. Craft stores and nurseries will have the larger ones, and layering different types is always lovely. Adding a little treasure can be fun, like a clear marble, a favorite rock/mineral/crystal, moss or lichen, idk, a cool stick? Try all sorts of things!