>What is Comiket?
Also known as the comic market, it's the largest doujin (indie/self published) gathering event in the world where people come to buy and share doujin products. This includes games, manga, erotic material, music, and other merch. It usually takes place in the Tokyo Big Sight.
>Can I shop for some of the products online, and if so, then where?
Consult the buyfag guide (buyfag.moe). Personally I use Melonbooks and Toranoana to buy doujins and music. There's also Booth.pm that some artists will use to sell stuff they won't sell anywhere else like dakimakuras or certain music CDs.
With that aside, what are you guys looking forward to? I haven't really looked at what games were coming out (no doujin game collection video has been made yet) but I'm mainly looking forward to some more Kancolle, Touhou, and Azur Lane jazz arrangements. Maybe I'll find some other gems too.