Early on isn't a good time to spend money promoting anyway, imo; people find your comic, see only a couple of strips, think "that seems promising, but it's too early to tell, I'll come back to it later" and then forget about it until they run into it again later.
I think one of your best bets early on can be participating in communities. The forum here, the forum on comicbookhour, the webcomicchat and comicbookhour hashtags on twitter, that sort of thing. And it's not just communities strictly related to comics, either -- my first comic years ago found a bunch of readers from folks in a D&D forum where I was a member, that liked my drawings of my D&D shenanigans, and checked out my other work.
Participate in communities, share your work when appropriate, engage with others when you can, but right now it's best to focus on making your comic the best it can be, since building an audience tends to be a really slow and gradual thing -- and having built up an archive of quality work will help keep potential readers once they find you!
