Well if this is your first, there's a lot of first timers as well. From my experience (Me being one of them), people tend to get overly hype and just do the randomness and 60% of the time, stupidest things. A friendly advice is to like, keep yourself calm, try not to go screaming up at people because they're in your favorite character cosplay. I say this because even though it's meant to take it as a nice way, it can be annoying/scary to some people, or other people may just be having a bad day and yelling wouldn't really help.
Expect a lot of wandering cosplays, events, and all those goodies. Hey, there's probably going to be one of those giant glomp circles again, but if you have a very fragile or semi-fragile cosplay, avoid those. Some people tackle extremely hard and can break something.
Remember to have cosplays or food that's meant for the summer. It gets really hot D:
There's a lot of perverts out there as well, so keep in mind who you talk to. If anything suspicious happens or harassment, report immediately, don't try and confront it if you don't know how. ALWAYS STAY IN GROUPS. It's safer that way, so try and make some new friends while your there so you know how to get around as well ^^.
There's a lot more about what to expect, but I think this would help give you a glimpse what kumoricon usually deals with:
https://www.kumoricon.org/forums/index.php?topic=18198.0Though it may seem there's a lot of cons, there's a lot of pros as well, so don't take it all to heart! Some people just got unlucky, however, it's nice to be aware, y'know? Check out "General Kumoricon" on the forums for more of these threads since there's also some advice there here and there.
NOW FOR COSPLAY.
I love to buy mine at CosplayFU since I currently have a amazing experience with them (While some have bad and might say otherwise). Their cosplay are pretty cheap compare to other cosplay stores, usually having sales and giving you discount coupon whenever you buy something. Did I mention free shipping? Yup, they have that too. If you buy a cosplay set, hey, bigger discount~. Most of their cosplays are pretty accurate in my opinion and you're always welcome to have changes made to it! just gotta tell them what (Like if you want a different color, etc).
My advice is that you USE their measurement conversion location on their measurement graph for the best fitting cosplay. I also suggest to at least add an extra inch to the measurement so it feels more loose, but that's your choice.
I buy my other cosplays at Hellocosplay as well and had a pretty good experience. However, it's a tad more expensive.
I have no idea where to buy contacts or lashes, so sorry D: .