Key things to consider are:

• zoom range: A wider range is more flexible, but very large-range “superzooms” often trade off image quality, especially near the wide and tele ends.
• maximum aperture: A larger aperture (smaller f-number) helps in low light and makes it easier to blur the background.
• minimum focus distance: This determines how close you can get and still focus.
• weight and size: Important if you travel, shoot handheld a lot, or simply don’t want a heavy lens.
• real-world image quality: Reviews and sample images are very useful, since specs don’t tell the whole story.

In general, lenses with narrower zoom ranges often deliver better sharpness than extreme all-in-one zooms, though convenience may matter more depending on your needs.

If possible, try borrowing or testing the lens before buying. Hands-on use can tell you much more than reading specifications alone.