Update regularily, practice and improve you art and your story, talk to your subscribers.
Be patient, and experiment with methods of building an audience. One method I advice strictly AGAINST is running around to individual people telling them to read your comic because then they're actually less likely to be interested. Most people percieve that as pushy. So in that case it doesn't matter if what you're promoting is good or not, they won't want to see it.
Building an audience takes time, I'll be 21 this year and I started out with 0 subs at the age of 14. I raised them by being persistent and submitting my art to up to 70-110 groups on deviantart In the beginning it did very little, then when I was like 16 maybe it would give me 2 new subs a day. Doesn't sound like it will actually build an audience, right? But only a lucky few people explode into 1 million subs or something overnight.
I've also done advertising on project wonderful before...
It's a tooooon of work for years upon years and many balls to juggle all at once for all those years in order to make a success out of it, so you have to really really want it. Many people are jealous of artists, singers and youtubers because they think we just chill in our beds most of the day, use some magical talenty-thingamajingy to create a piece in the matter of 30 minutes, toss it out into the web and then get tons of people wanting to bang us as we go back to eating chips and chocolate while watching how I met your mother. It's probably also why a lot of people don't think it's a real job. In reality, it's one of the jobs that requires the most education, experience, social communication and persistence for the longest damn amount of time before life goes "k, you're hired".
But, the good news is that one day that hard work will start to pay off. As I said, it's unlikely to happen soon or overnight, but I've turned that 2 subs a day through constant manual advertisement, into 100 new subs a day simply by having the coolest people ever reading my comics. I've been working almost every hour I could spare for the last 7 years to get here, but hell it was worth it.