The first step is kinda of an obvious one. To create enough content to make your comic worth promoting.
After that the key is to figure out your audience to choose the most effective media to reach potential readers.
Now, to build engagement, there are many ways. There are content related tips, like dropping foreshadowing and hints of future events, which will encourage readers to build theories.
Another one is to build up a solid pacing. If you make every update entertaining and/or relevant. (hopefully both), people will have something interesting to say about the page aside from "first".
Now, if we go to the marketing side of things, to engage with the audience one can make little contests. Make sure you have an interesting prize that is attractive to them.
Other tricks is to create extras like a Q&A session after important chapters to get readers to know more about the characters. Other examples could be sharing guest art and fancomics (with the artist's permission)
Crossovers are a posibility, but is important to choose wisely who you work with. I had some cool crossovers because the creators i was working with were familiar enough with my comic's character and we worked well together.
Hope this helps!