Topwebcomics looks like an older site but a LOT of readers still frequent the place to find new stuff to read! When I asked my readers how they'd found my comic, a lot of folks mentioned that they weren't entirely sure, but they know that 90% of the time when looking for a new comic they'd go look at TWC.
I definitely recommend registering for an account (like Emevsa said, this is free!) and adding your webcomic to the listing. Once you do that, you can send your readers a link to vote for you comic, which they can do once a day. If you climb up in the rankings, it's more likely that new people might find your stuff!
For the "vote incentive" where you can put a "thanks for voting" image, I just do silly scribbles of extra scenes with my characters or some bonus info about things that didn't come up in the comic, and I switch out a new "vote incentive" every time I update my comic, which encourages my readers to vote at least once a week to see the new incentive.
As to getting advertising on the TWC site, I've never gotten ads there myself, but I've heard a lot of good things, so it's absolutely on the to-do list for me when I have the cash for it. This article has some solid numbers if you're interested!
