The main Inca trail has a limit of 500 people starting on it each day. This sounds like a lot, until you realise 300 of those are porters! So it's quite hard to get it. My brother had to book months in advance.
However, there are several other trails. Salcantay is meant to be higher, less ruins but more scenic. I'm told the Lares trail goes along the river a bit more. And you still end up at the same place. They're all much easier to get onto, and even National Geographic has an article on the top 6 alternative trails.
Wikipedia has a short piece on the Salcantay Trek, if that interests you.
Finally, there's the 'easier' option - where you take a bus to Ollantaytambo and then a train from there to Aguas Calientes. Then the next morning at 3-5am, you get a bus up to the site. You'll want to be on the first bus to ensure you get one of the limited tickets to hike up Huayna Picchu (the tall peak you see at the back in most pictures of Machu Picchu).
A friend did the Salcantay, raved about it. My brother did the Inca Trail, said it was the hardest thing he's ever done in his life. It all depends what you want, and how you're affected by the weather, the altitude and the experience.
But the best part - no matter which way you get there, you're ending at Machu Picchu. And that's fantastic!
In terms of reserving, it's hard to tell. As many people and websites will tell you:
Peak tourist season corresponds with the dry season in July and August
when the nights are cool and the days are generally dry. January and
February are the rainiest months, and many people visit the area in
November and April to avoid the heaviest crowds.
So on that theory, April might be quite busy. If you really want to do the Inca Trail, it's your 'once in a life' thing, then reserve it and make your schedule slightly less flexible.
If you, however, have some flexibility, well you could risk it, but stand the risk of not getting the main Inca Trail. But there will always be one of the other trails available, at least.
