Unlike other marathons, the registration process for the Boston Marathon doesn't just involve paying, signing up, and running. You actually have to qualify to be able to run it. And they also limit how many people can actually participate each year. First off, to qualify, you have to run a qualifying marathon, meaning it's a marathon with a race route certified by the USA Track and Field. You also have to run this qualifying marathon under a certain time. The qualifying standards for men and women of various ages are also different, so you will need to check on what your specific qualifying time is based on your individual age and gender. (For instance, men ages 18-24 must run their qualifying marathon time in 3 hours and five minutes or under.) Finally, these Boston Marathon qualifying races take place within the year prior to the Boston Marathon you are aiming to run.

So training for the Boston Marathon requires training and performing well in a marathon BEFORE the Boston Marathon- which means you will have to begin training sooner than you had probably anticipated. You'll need to pick a qualifying race that fits your running abilities, and a training program that puts you on target to perform well enough on race day that gives you a Boston Qualifying time.

As far as training for Boston itself, you need to focus on endurance first. You have to first have your base built up, and need to just get through the 26.2 miles. Learn how to pace yourself, and run smart. The weather in Boston in April can be hit or miss - it might be sunny and warm, or very cold and windy. Dress appropriately, run the tangents, and hydrate and fuel well. And when you run your long runs during training, be sure you throw in some tough hills. Heartbreak Hill at mile 20 of Boston is infamous for breaking down the spirit of runners who have given their all for the first 20 miles and are starting to lose momentum but have a huge incline in front of them with another 6 miles left. Avoiding injury is all about proper and thorough training!

Sources
https://www.runnersworld.com/ask-coach-jenny/how-to-qualify-for-the-boston-marathon
https://www.runnersworld.com/boston-marathon/2018-boston-marathon-registration-faq