Applying varnish, paint, or seal your structure.
Treating the wood against the elements is a must, especially if the structure will be outdoors. Even if you're building indoor the structure for indoor use, finishing it will be a safeguard against everyday wear and tear, and beautify the structure. Almost any varnish, stain, or paint will work just fine for stairs. You can also use non-slip paint or put non-slip adhesive paint on structure.