My day job is in advertising, and we're always doing social media stuff. Here's some basics to help you get started:
Pictures and videos will always help you get more engagement with your posts (ie. more people liking and retweeting)
Videos at this moment I think have a limit of 1 minute. I use twitter to post previews of my youtube videos. Make sure you video is attractive enough without sound to keep people's attention
Tweet a mix of your own stuff and retweets of what you are interested in? For example, I'll retweet things I find interesting like other comics or Eurovision/F1 news, because that's what I'm into. For one, it shows that you participate in the world around you. People like to see that you are about more than just your own stuff. It might seem counter intuitive, but retweeting tweets you find funny/interesting/important/whatever actually can help build your own personal brand because you're building a personality for your account.
With #webcomicchat, it starts with answering the 5 questions in the session, but sometimes side conversations can break off in regards to one of the questions. If you see one you want to join, don't be afraid to chime in with something in regards to the conversation! Everyone is very nice. After a few weeks, you'll start to become familiar with people and it will get easier to talk to them.
Also, #ComicBookHour is a nice community in a similar but more professional vein. I chime in their from time to time as well.
For more information, here's a pretty thorough guide to get you started. It's from a marketing perspective, but I've found that marketing a brand is the same thing as marketing your comic. Good luck! And feel free to follow me @MelissaJMassey on the tweets and you can ask me questions there too.