Look online for some local conventions and start small if it is your first time selling. I've sold mostly at college conventions where the booth costs no more than $100. When you find one, send them a nice email or fill out their vendor or artist alley application. Do so very early!
It's a good idea to try to keep your costs down at your first convention, so decide on one or two things to sell (shirts and buttons for example) and see what sells best so you know what to bring to future conventions.
If you are selling at a very specific convention, do your research and make products that appeal to the audience. My biggest profit was at a My Little Pony convention, where I learned how to draw ponies for quick sketch commissions at $10. With those commissions I made $300 by the end of the day!
You are going to be confused about how many prints to bring, and I always am as well. It's easier once you have done a convention already and know which of your prints sell the best. prints of popular characters and fan art will usually sell the most. Make sure to have original art at your booth as well, many conventions have 50/50 fan art/original art rules. Look up the rules of whatever convention you are attending!
You can make a booth with a nice tablecloth and either wire storage cubes, PVC pipe, or a combination of both! I use wire storage cubes from Amazon to build around my booth and then attach my best selling sample prints to the front. I also keep an 11x17 binder so people have something to flip through and can see all of my prints. Don't forget to put some free business cards out on your booth as well!
Make sure to check out examples of other booths before creating yours!
Where to get prints
Moo.com is the most recommended website I've heard of to get prints online, but if you are cheap like me you could print at your local college or good print shop. Just keep in mind this is not the place to print your more sexy artwork, as that gets awkward fast. Never print somewhere like Staples as the quality has always been horrible in my experience!
At my college, prints are under a dollar each, which I then sell for $15 each. Aw yeah! If the convention is particularly small I will sell for much cheaper, so more poor college students can afford the work.
Don't forget snacks, because unless you have a trustworthy booth mate or helper there you will not be able to get up to get food for many, many hours. Bring paper to draw commissions on as well if you are doing them. Different art materials never hurt to bring, I once had someone request a commission drawn on a CD and I had to get some sharpies to complete it!
Invest in getting a card reader, at multiple conventions I have been the only one who takes card and it helps SO much with profit! :) get the Square or Paypal one, they reimburse you so it is free and then they take a percentage of your profits but it is still super worth it to get.
Get there as early as possible to set up! this part is always the most stressful for me. I put together my display as fast as possible, and I often try to bring pieces of it already put together. I lay it out looking all nice and keep my money, phone, and some prints on my lap. I try to keep one of each print at hand and the rest underneath my booth in a storage container, organized with sticky notes with the name of each print on them (like a bookmark!) This way they are easy to find and you can quickly give someone a print and exchange money!
Before setting up prices, I like to look around at other vendors booths to see what their prints are selling for and adjust mine accordingly! If everyone is selling prints for $15, I don't want to be selling at $25 unless my art is much better than theirs and vice versa! Make sure your price signs are super visible, many people are too shy to ask the price on a print and will pass by without asking.
Another fun strategy is to include some deals in your pricing, like Buy 3 get one free! or something similar. This helps drive funds like no other, and your customers will love saving money.
as always, take payment upfront. Once at a convention I had the same person come up to me multiple times, saying how my art was too expensive for them but that they really wanted it. eventually they grabbed some of it and said they would take it now and pay me later if I gave them my address (ha!) I politely declined, although I wanted to go on a huge rant.
Convention goers are often annoying, and will place food or drinks on your table or sneeze on your artwork, but try your best to be polite to them as well as your booth mates and neighbors, as you can get awesome help from a nice booth mate (bathroom breaks! snack sharing!)
Happy convention-ing and I hope this helped some of you out! If you have any questions, please submit them to the comments below!