A short sleeve jersey is the best starting point for most cyclists. It's the most versatile option—you get maximum ventilation for warm weather, and you can easily add arm warmers when temperatures drop. This layering approach gives you the flexibility of a long sleeve without committing to one weight of jersey.
Short sleeve jerseys work well for road riding, mountain biking, commuting, and general training. They're the foundation piece that most riders reach for on the majority of their rides throughout the year.
The one downside is limited sun protection on your arms, but that's easily solved with lightweight UV arm sleeves or sunscreen. If you're building a cycling wardrobe from scratch, start here—then add a long sleeve or thermal option later for cooler days.