The official Philmont packing list is available at:
https://www.philmontscoutranch.org/philmonttreks/whattobring/
More detail on the big items is here:
https://www.philmontscoutranch.org/philmonttreks/shakedown/
For all items, consider weight. You don't have to buy expensive ultralight equipment, but be aware of weight and choose lighter weight options if available.
For sleeping bags, I would recommend a 20 degree rated down bag with water resistant treated down. Down bags compress better than synthetic, and are lighter. I recommend the Kelty Cosmic Down 20 which retails for about $150 but often goes on sale.
For packs, look for an internal frame 65-80 Liter pack. You want to take your time and select a pack that fits you well, and I would recommend going to a store with backpacking experts such as REI or Midwest Mountaineering. Try on at least 2-3 packs, and walk around the store with each while it's loaded with weight. You want to make sure fits you well since you'll be carrying up to 40-50 lbs for many miles each day. Take your time, and choose based on how it feels rather than the brand. Many scouts have narrow hips, and sometimes a women's pack will fit better. You can rent a backpack for $30 at Philmont if you don't want to buy one. For scouts that are still growing, the Osprey Volt 60 and 75 are very adjustable and worth considering.
We will be backpacking during the rainy season at Philmont, so good rain gear is important. Your rain gear doubles as a wind resistant layer for warmth, so rain pants are also required. Make sure your rain gear is "water proof" and not "water resistant". Frog Toggs are not durable enough for Philmont.
For clothing, avoid cotton. Choose synthetic shirts, shorts, and pants instead.
I would not recommend buying hiking boots until Spring because your feet may still be growing and you want to make sure the boots fit well at Philmont. REI has a big sale around Memorial day, and that is a good time to buy boots. Merrell Moab 2 Ventilator Mid Boots are relatively lightweight, inexpensive, and are also recommended for Northern Tier BWCA canoeing.
The official packing list specifies 4 32 oz Nalgene water bottles, but you could consider a hydration pack instead. I recommend the Platypus 3L Big Zip SL Reservoir. You can also consider lighter weight Playpus SoftBottles or even empty SmartWater bottles instead of Nalgene bottles.
The only item not on the list that I would recommend is a seat pad. After a long day of hiking, having something soft to sit on is really helpful. The Thermarest Z-Seat is a good lightweight option.
The Tooth of Time Traders is Philmont's on-site outfitter store, and their website is at:
http://www.toothoftimetraders.com/
Everything sold at this store is tested and appropriate for Philmont treks. You don't have to purchase from this site, but it will give you a good list of products that work well at Philmont.