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.
Regular hillwalkers figure on taking 5.5 hours to make the round trip to the summit. It could easily take more. As well as needing stamina and fitness for the climb, plenty of people struggle with their knees and joints on the long descent.
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 parking space (though this can fill up in the height of summer).
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 "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.
There is a small survival shelter on the summit, elevated 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!
RAF Air-Sea Rescue offer free rides in a big yellow helicopter, but it's considered bad form to call upon them.