Most frequently, and especially in Canon-centric groups, the answer will be "Get the Nifty Fifty!"
On its surface, this is solid advice! Canon's EF 50mm f/1.8 is a lot of bang for the buck. Compact and lightweight, but sharp and with a fast maximum aperture, the "Nifty Fifty" would seem to be a no-brainer. Plus the 50mm prime has been a staple in any kit since the dawn of 35mm photography, since its ~40° field of view closely approximates normal human vision.
But when you buy that 50mm lens and you slap it on your beginner DSLR, which has an APS-C sized sensor, commonly referred to as a "crop sensor" since its small size crops off the outer parts of the lens's image circle, you wind up with a field of view that is much narrower.
On an APS-C sensor, a 50mm lens has a field of view equivalent to a 75mm lens (or 80mm on Canon's APS-C sensor, which is slightly smaller than everyone else's). That's closer to a "short telephoto" or "portrait" length. That's why the little look-a-Leica crop-sensor Fuji X-E1 that I use for playing hipster street photographer pretty much permanently wears a 32mm f/1.8 Zeiss Touit, which equals a 48mm in full frame terms.
But that's a middlin' spendy aftermarket lens. I bought it used, but even used, it was "used Glock" money. It doesn't really fill the "Nifty Fifty profile of being cheap & cheerful while delivering bang-for-the-buck.
Some manufacturers remedy this by selling dedicated crop sensor equivalents to the "Nifty Fifty" for their own systems. Fujifilm themselves make a decent little 35mm f/2 that sells for about two hundo, and Nikon (who use "DX" to signify APS-C lenses and cameras) have a crop sensor "Nifty Fifty" equivalent in the excellent AF-S DX 35mm f/1.8G.
Canon, sadly, doesn't offer anything like this in their crop-sensor EF-S mount; the closest is the 35mm Macro which is, at $300 and f/2.8, too spendy and too slow to really be a "Nifty Fifty" equivalent.
Since I'm primarily a Canon shooter who only dabbles in Nikon for fun (and mostly old film or FX Nikons at that) I've only played with two Nikon DX lenses, the 35mm f/1.8 and the 18-200mm f/3.5.-5.6 VR II, and I've already come to the conclusion that, compared to Nikon, Canon never really gave much of a toss about crop-sensor glass.
Apparently once their 1Ds and 5D hit the market early in the millennium, with their big 35mm full-frame sensors, Canon was like "APS-C? Pfft. Let them eat cake."
Both EF-S and EF-M have a solid little wide-angle pancake prime, a decent macro and... Well, the EF-S 18-55mm f/2.8 is nice, and there's that new 32mm f/1.4 for EF-M that's almost $500, but otherwise crop-sensor Canonland is an uninspiring plasticky wasteland.
Canon expends a disproportionate amount of effort on their high end L lenses, but since their marketing seems largely designed to funnel people toward them red rings, it makes a certain amount of sense.
Most people who buy a Rebel or EOS M just to take vacation or family pics generally won't know or care, but a certain amount will get bitten by the photography bug and migrate upward. It'd be nice if there were more higher-end EF-S/EF-M lenses for them to go to rather than Canon putting all their glass eggs in the full frame sensor basket.
.
