The first 100 is always the hardest, but once you get your stride and people start to notice you, it only gets faster and better as time goes on. I do recommend getting some social media accounts and having a nice large area to attract new viewers, but don't spam - It only turns people off. It took me about 4 months to break 100, but have since gained another 400 in the 6 months following and set to reach 1000+ by the end of the year. Just keep working on your channel, think that rather than 15+ subs supporting you, you've got 15,000+ and you'll get a long way! (It's how I've kept uploading EVERY single day despite having a fairly small channel. I enjoy doing it, so I just do it!)
If you need a few ideas on how to get it all up and running, I've got links to all of my social media and website in my signature.