Seconds Out is a really good place to start. If you want to start with a studio album, Trick of the Tail would be my recommendation. My favourites are Foxtrot, Nursery Cryme and Selling England by the Pound. ...Lamb... is considered their creative peak, but it would be like getting into Pink Floyd by getting The Wall -- great album, but not indicative of their spacier/space-rockier moments at all (DSOTM, Animals, WYWH, "Echoes", "One of These Days", etc.). Genesis are very accomplished songwriters, with a proggy bent, so that's what you should want to hear, methinky.