As the highest point in Great Britain, Ben Nevis is a hugely popular hill to climb. It is 1,344 metres (4,409 ft) above sea level, and the start of the walk really begins right by the sea so you'll walk every foot of those 4,409.
Figure on taking 6-9 hours to make the round trip to the summit. As well as needing stamina and fitness for the climb, plenty of people struggle with their knees and joints on the long descent. Waterproofs, boots with ankle support, walking stick, and a packed lunch are a good idea. There's no cafe at the top of this one!
The two usual approaches are from Achintee Farm or Glen Nevis. Both are over a mile out of Fort William, and of the two, the Achintee route is less steep to begin with (they merge pretty soon) and has some free parking spaces (this fills up early in the height of summer), while at Glen Nevis visitor centre there is more parking but you have to pay.
The usual route is the Tourist Path aka The Pony Track. This runs from the end of the lane at Achintee Farm, though there is an access route from Glen Nevis, and heads up in a series of zig-zags to the summit on a broad, obvious path. It's a relentless slog, well trod and eroded by tens of thousands of people every year, though path repair work is currently underway. The path heads steadily uphill, a couple of small zig-zags, then curves left at a cleft in the hill before levelling out at what is known as the Halfway Lochan. (It's slightly before half-way but who cares, it's a morale boost.) Then are the zig-zags proper, a series of eight switchbacks in the path, then a final straight up the final slopes of the hill.
The "interesting" route is to ascend the outlying hill Carn Mor Dearg (pron. Jerag), and traverse the arrete to Ben Nevis proper, but this one is strictly for equipped, experienced mountaineers.
At this altitude the temperature is considerably lower than in the valley where you start out, plus you need to factor in for some wind chill, meaning that you need warm clothing. Snow is common on the summit even in midsummer.
You're unlikely to get lost until you reach the summit. A direct walk across the summit to the cairn would send you tumbling down a gully, which becomes a hazard when filled with corniced snow which may look safe to walk upon. Overshoot the summit or lose your bearings and you may fall off the mountain, such as via the dramatically named Five Fingered Gully. These issues don't sound too much of a problem until you realise that even in summer the summit is fogbound more than half the time.
There is a small survival shelter on the summit, known as the Snoopy Hut, built on the ruins of the old observatory to avoid it becoming snowbound.
Many visitors arrive as part of a Three Peaks Challenge event, particularly in the middle of summer when there's plenty of daylight. This involves climbing the highest mountains in Scotland, England and Wales, and can cause chaos. Charity organisers are asked to follow a code of practice, which limits group sizes, access hours, timed challenges, etc. The biggest risk if you're a participant is breaking your ankle if you're running downhill - which also helps trash the paths in a scenic area!
There's a distinct lack of toilets on the route, or even handy pathside bushes. On descent your first toilets are at the Ben Nevis Inn, at Achintee, but they're only open during pub hours. At Glen Nevis visitor centre there are also toilets. The only slightly private spot during the walk is by the Halfway Lochan, 100yds down a spur path to the north from the route, where a large rock affords you a little privacy. But it gets pretty gross back there.
RAF Air-Sea Rescue offer free rides in a big yellow helicopter, but it's considered bad form to call upon them.
Fort William is at the foot of the hill and is the obvious base for food, drink, accommodation, equipment shopping.