Setting the Scene
Whenever your Player Characters enter a new area or environment it is important to thoroughly describe the area, painting a mental picture for the PCs to explore. The more detailed your description, the more accurate the picture your PCs will have, and the more smoothly your game (and especially your combats) will run. Below are some aspects you should consider when setting your scenes.
Use the Five Senses
When setting up a new scene, it's easy to overlook our secondary senses. In addition to describing what the character's see, consider what they might hear, smell, feel, or even taste. Engaging all of the character's senses helps to create a fuller, more realistic experience, and can give them a clearer view of their surroundings than a simple visual description. Consider the following two examples for comparison.
Example 1
As you come around the tunnel's final bend, it opens up to a large cavern, where an underground waterfall towers above you, feeding a large lake that spans most of the caverns area. The walls glisten with moisture and mist hangs in the air at the waterfall's base.
Example 2
Long before you reach the tunnels final bend, you hear the cacophonous sounds of rushing water. The air feels damp and smells faintly musty. As you turn the corner, the tunnel opens up into a large cavern where an underground waterfall towers above you, feeding a large lake that spans most of the caverns area. The walls glisten with moisture and you can feel the mist hanging in the air at the waterfall's base. As you approach the lake, the water smells fresh and clean. When you take a sip, the water is cool and refreshing.Generally speaking, the two examples describe the same area, but the second is much more detailed and incorporates all of the character's senses. In the second example, the character also gets a notion of what might be coming next, before they even have sight of the waterfall cavern. This helps to create the feeling of a living breathing world, instead of a single isolated area.
Be Detailed
In addition to describing the large and obvious features of the area, consider more detailed aspects such as lighting and the size and height of the area (if enclosed). For outdoor spaces, consider the type and density of vegetation (i.e. lush jungle, thick forest, sporadic chaparral, or sparse desert), rock and landscape features, weather conditions, and whether the PCs are on a well traveled path or trailblazing. You should take note of the PCs Line of Perception based on the environment (i.e. dense trees, inclines, and rock formations could block sight, while waterfalls and rushing rivers may block hearing).
For indoor spaces, consider the type and style of any furniture, decor, or other objects that might hint at a room's purpose. The style and decor of an area can tell the PCs a lot about the inhabitants (current or former). Take note of the cleanliness of the space, exits to the room (doors, windows, hatches, trapdoors, etc), and lightning conditions (natural light or torches, candles, sconces, etc). Cleanliness can hint at the status of the location (i.e. dust signifies its not been used recently, unless there are disturbances in the dust) as can lighting (someone had to be around to light those candles and torches, unless they are magical).
Set a Tone
In addition to providing your players a mental image of the scene, you can set a feel or tone for the scene (and events to come) by using descriptive words the evoke a deeper or emotional connection. Words like horrific, dilapidated, chilling, cozy, comfortable, or quaint can be used to describe not only the look of a scene, but the "feel" of it. You can also you tone to give a sense of foreshadowing of things to come, such as a sudden "stillness" in the forest. This can be especially useful with interactions with groups of NPCs, allowing the PCs to get a sense of the mood permeating the characters in the scene.