Oh goodness where to begin...
Last year's Everfree was my first convention ever myself, so let me give you a few tips I wish someone would've given to me. The main two being SLEEP and EAT. Breakfast is the meal of champions. If you get all your food from the 7-11 across the street, you will have stomach problems by the end of the con, so keep that in mind. Also, I know how much you want to stay up for all of the dance party and still get up for the morning VA panel, but trust me, it's not worth it.
Have a budget that you set in advance, and don't buy above that budget. The vendor's hall was chock-full of amazing stuff, but I limited myself to what I could afford and thanked myself profusely for it later. The same goes for food, like I said you don't want to eat at the 7-11 for every meal but you also should avoid getting room service every night.
Waiting in line is one of the best experiences at a con! Don't feel like you need to see every single panel, they will all most likely be available afterward on YouTube, or if you have a laptop you might even be able to stream them while waiting in line for the next one. That being said, be sociable! Everyone is super-excited to be there, so they all will be extremely willing to have a conversation with you about almost anything pony-related. While it can be fun to listen to the voice actors we all know and love to talk about working on the show, the main reason we're there is the community. Don't let it fall by the wayside!
Keep track of your belongings. Have a backpack if you're carrying around more than two things on a regular basis, it makes everything easier. If you're rooming with someone, it might be best to have your valuables with you instead of in your hotel room. Keep your room key with you at all times.
As RainbowShy said, hygiene is important. Everyone will thank you for showering. Get your hands on some vitamin C or some other immune-system booster. Also, coffee is your friend, it is Seattle after all.
Depending on whether you're flying/driving/riding in or you live in the area, there's also some logistical tips I could give you, namely that public transportation is your friend and Seattle's an awesome city. Also, if you're thinking about cosplaying, first of all you definitely should and second of all make sure everything about your costume is secure. I had some fun with my fake mustache and the ribbons on my hat last year (as Rarity's dad, picture in my avatar). Don't be nervous about dressing up, I had quite a few nice compliments from a lot of people, ranging from fellow cosplayers to random people in the 7-11 who said "Nice mustache". Like I said, 99.9% of the people you'll meet are incredibly nice.
Lastly, don't be stupid. If con staff tell you not to do something, don't do it. Be extremely kind and courteous to everyone around you, especially the kids and the VIPs. This is a family convention, and it revolves around a show for young girls. Don't lose track of that.
I'm sure you'll have an absolutely fantastic time! Everfree was one of the best experiences of my life, and I'm so glad you're going to get to experience it as well! See you in July!!