Here's some of my thoughts on tabling, having done conventions as a craft artist and a print/comic artist for about 4 years now:
First of all, consider starting at smaller conventions and art shows. There are often local events where you can get a table for under $50, and you won't have to account for all the extra costs like travel and hotel. That right there will take a lot of stress out of your first tabling experience!
Also, you probably won't make a lot of money at your first con. If you can break even, awesome! But you might not, hence why it's good to start small. If you do make profit, huzzah! You can put that toward your next show.
There's a lot of variables at a con that you can't control, so focus on what you can: Your product, your presentation, and your salesmanship. Ideally you want to have your stuff in the ballpark of other AA artists in terms of quality, and a presentation that stands out to passerbys (so ideally a vertical display people can see from afar-- avoiding flat table syndrome.) But I think the biggest component of this is gonna be you.
I've seen people with fantastic artwork but crappy sales because they kept their head down all weekend and didn't engage with people (I've even seen artists sleep under their tables @_@;). Even if your artwork doesn't measure up to others, even if you couldn't afford a nice display... if you're friendly and engaging, and you greet everyone who approaches your table, you'll do all right. At the very least, people will remember you for talking to them.
And unique to comic artists: Develop a compelling pitch! If you're selling comics, people are going to ask you what your comic is about. You need to be able to tell them in like... 10 seconds tops. Develop a logline/elevator pitch, and save the more detailed one for if they seem interested and want to know more. Many times, I've asked a person what their comic is about, and they launch into a long complex retelling of the plot... and I'm totally lost.
I have pretty bad anxiety, so I hate getting this question--- which is why I developed a short go-to pitch for when people ask what my comic's about. That way I can use it somewhat reflexively instead of letting my brain go into panic mode, resulting in the aforementioned rambling plot retelling. That said, this is probably the hardest part about selling a comic to someone who isn't familiar with your work, but if you can nail it down, you're gonna have a much easier time.