Even for people who can smell, the sense is rarely precise enough to describe save by way of analogy. Someone particularly interested in cooking or food in general might train themselves to be able to pick out and recognize the specific elements making up a scent, but for the general population, if you're not simply saying directly what something smells of, relate it to the general categories of its composition. Something might smell metallic, earthy, floral, or oily, for example. Wet materials usually, although not always, stand out over dry ones, and hot or even just warm ones over cold. The longer something has been part of an area, the more it will be part of the background scent, and faintly noticeable even after cleaning. And being imprecise is fine; most people will say 'the smell after rain,' not 'petrichor.'
Although the specifics of what falls into each can vary from person to person, people will commonly also notice whether things smell fresh (clean, as in air untainted by perfume or vehicle exhaust, but also alive; a hospital, laboratory, or other sterile location would not generally fall into this category) or stale (dry, left too long; places that receive insufficient ventilation and foods that go bad without rotting). Rot or (especially in older media) foulness is distinct from either; it's the wet, alive kind of bad smell, produced by most kinds of decomposition (and by extension, excrement and the like). It's also far more noticeable than the other two, and will overpower most other scents present.
How much can you taste? The two senses are strongly linked when both are present, so almost any adjective you would apply to a taste can safely be applied to a scent. Acrid smells, such as from strong cleaning substances or burning plastic or rubber, will sting the nose in the same way spice does the tongue.
Another thing to keep in mind is that, as with touch, constant exposure will dull one's awareness of a scent. Much as one is not constantly aware of the feeling of one's clothing, the smell of someone's own home or a perfume they wear daily will be almost unnoticeable. Small changes to otherwise familiar environments will tug at one's awareness much like new clothing. The smell of an unfamiliar location, however, will be primarily noticeable when first entering it.
Finally, as OldBat alluded to, for most people, scent is strongly linked to memory. The majority of scents won't be connected to anything in particular, but it's common for someone to have specific things they can't smell without being reminded of past events. A particular recipe might, for example, not smell just 'of cookies' but 'of weekends at Grandmother's house.' Not to be overused, but good for adding some emotional association to a scene.
Charles Staats brings up another excellent point:
Note also that association plays a significant role -- I personally dislike the smell of fresh cut grass, for instance, because I associate it with itchy eyes (because of my allergies). Especially relevant here are the smells of coffee, alcohol, and tobacco -- these will typically smell awful to someone who does not use them, but good to someone who does. (Or so I understand -- the only one of these I can personally attest to is coffee.) Tobacco smoke especially is interesting since many ex-smokers detest the smell.