85mm or 50mm fast prime
$$$A fast prime is how you separate a face from a messy background. 85mm (or 56mm on APS-C) is the classic; 50mm is cheaper and better in small rooms. Either one at f/1.8 will out-bokeh a kit zoom at f/4.
What to look for: 50mm or 85mm full-frame equivalent, f/1.8 or faster. Eye AF works on almost any modern body with these.
Skip it if: you already own a 50mm f/1.8. Use it. An 85mm can wait until you know you want more compression.