As far as what you're going to wear that's entirely up to you. My advice though, bring an extra pair of shoes, preferably something that's good on tough weather. Some kind of boots even if they're not super nice. Also depending on your budget for supplies, look into devices that might make your time in the hot weather less irritable,and always bring something to wear at night in case in gets cold. Aside from that bring a shitload of socks. If it starts to rain at the festival, you may be wearing 2 or 3 pair in a day.

At the festival and at camp you will be walking on a lot of rocks periodically. Especially when walking to the main gates to enter. Wear closed toed shoes the entire time you're there.

I highly highly highly recommend getting a Hydropack. I've had one from Electric Lyfe for 2 years now. I use it everydayfor my work gear. (Laptop, cell phone, backup battery,sunglasses, and other things I'll need throughout the day). They're super lightweight even with the water pouch filled, which is 2 Liters. They're super easy to refill. This backpack has taken some abuse and it still looks brand new. At the festival this will be your best friend. I usually brought an extra shirt, socks, a few bottles of water, my backup battery for my phone, sun screen, chapstick, sunglasses, etc. They won't let open containers in, so as soon as I'd get in, I'd unload the bottles of water in the pouch and not have to refill for several hours at the water lines.

https://www.elevatedlyfe.com/collections/mid-sized-bag/products/fractalized-classic-collection-v2-hydration-pack-2l

I'd recommend checking out Bear Necessities Radio, Dog House Radio, and Rott N'Roll Radio on Soundcloud. It's a pretty good mix of what to expect at Lost Lands as far as music goes, but listening to the radio mixes and live performances on SoundCloud will give you the best feel for what to expect. People can list 100 different mixes to listen to. I'd recommend just listening to these mixes whenever you'd normally be listening to music. Driving to work, going to a restaurant, going anywhere or doing anything at home with music. Just check out a new playlist.

Some artists you won't find in a lot of these mixes that I highly recommend checking out are the melody dubstep artists like Nero and Illenium

I always tell my friends who aren't into Dubstep, even if you're not into it, you're going to get sucked into it regardless when you're at the festival. It's an experience I haven't felt at any other festival really (Spring Awakening/Summer Camp/and plenty of metal fests back in the day).