Alot of good suggestions here, all good lenses. I feel though that where your at as a photographer is important in deciding which lens/lenses to get. Also you haven't told us your budget.
Wildlife usually means 200mm at the very least, some would say at least 300mm. Now that means either paying big money(£600-£2000), or getting a lens with a narrow aperture, which will in practice let in about a quarter of the light of a wide aperture lens (ƒ/2.8 vs ƒ/5.6), meaning either quarter the shutter speed (risking blur) or bumping the ISO up 2 stops (reducing IQ and adding noise). In decent light these lenses are fine and produce good quality images, but in lower light they struggle to focus and get usable shutter speeds. Decent wildlife/sport teles are always expensive so are out of reach of most enthusiasts initially. Though worth saving up for if you are in any way serious about WL and sports.
Portraiture is usually the 50-135mm range although can extend either way,
depending on particular style. A wide aperture lens is good here both for the added light/shutter speed and for having a shallow depth of field. As you are new i think the 50mm 1.8 would be a perfect lens for you as its great value. If you are serious about portraits, reportage or any other low light work where you dont need a particularly wide or long lens, then its an ideal lens. It probably the cheapest way of getting a 'fast' lens too.
If you want to have flexibly, then you'll need either multiple lenses (not all wide aperture primes are as cheap as the 50mm 1.8, so i think having a full set of primes is probably out of the question) or get a zoom lens to go with it. Seeing as you are into wildlife id say the 55-250 would be perfect to go with the 50mm 1.8. But if you want to do landscapes or are gonna be shooting in tight spaces then you'll want a wide to go with it aswell or instead of. Theres the 18-55 IS which is like the little brother to the 55-250 IS and is good value for money.
Adey