Your lens shapes your photos more directly than your camera body does.
Focal length determines field of view: wide-angle (14-35mm) for landscapes and interiors; normal (35-70mm) for versatile, natural-looking shots; telephoto (70-200mm+) for portraits with compressed backgrounds and subjects you can't get close to.
Prime vs. zoom: primes (fixed focal length) are sharper, faster, lighter, and cheaper than equivalent zooms — at the cost of needing to move yourself to reframe. Zooms trade some of that for flexibility.
The best-value beginner upgrade: a budget 50mm f/1.8 "nifty fifty," often under $150 — wider aperture than most kit lenses can match, and it teaches you to move your feet instead of your zoom ring.
Maximum aperture (the second number on a lens spec) determines light-gathering and background blur — kit zooms typically narrow as you zoom in (f/3.5 at 18mm → f/5.6 at 55mm), which is why prime lenses often noticeably outperform them in low light.