Get a 100% silicone dildo that's softish (probably not the Fetish Fantasy series, they're very stiff), with a bit of a head and a curve. Don't get much texture (it can be irritating), and get a dildo that's big enough - in general I find this to be 1.25+ inches in diameter and 5.5" of insertable length - but not too big. Talk it over and figure out how much realism you want in the dildo, and if there's any question go for less rather than more.
Medium Silk seems to work good, as does the Tantus Curve.