Glad to hear that you are going to give fly fishing a try! Great advice thus far has been given, and once you start catching fish via the fly rod you will receive a thrill that is hard to describe! You can get very creative with the overall process....and take it in whatever direction you see fit. Little fish or large fish hardly matters...and as you hone your skills it will become one more exciting way to expand on your passions for fishing and spending time outdoors.

A couple of photos to inspire while you prepare!

Best regards!

Can't say that I have ever bought a "package" of flies.
Go to Sportsman's Warehouse, Fisherman's Marine, Fish Field, or a local fly shop and chat with someone familiar with the water you intend to fish. They'll help you determine what to buy, based on "dry", "wet", fish species, etc.
My favorite, this time of year for trout of all varieties, even summer steelhead, is to use a foam hopper with a beadhead Prince Nymph or Copper John on a 12"-24" dropper..... covering top and "bottom" in one offering.
A B.C. Canada guide named Brian Chan (YouTube him) is the king of nymphing for trout in lakes, explains quite well.
I wish I could tell you what ALWAYS works, but if I could, I wouldn't have 5 or 6 (+?) boxes totally stuffed.
It's easy to get carried away when you stand in front of a display of a hundred+ flies and each pattern is available in multiple sizes AND multiple colors (which sometimes is the key...go up or down a size or two... go from green to black, orangey pink to orangey red......suddenly they like it, go figure !.... fickle little bastards, they are !)
Start slow and keep asking EVERYONE as to what is working....this week.... next week might be totally different.
And of course, you can get a tiny net (Dollar Store kid's section), do some wading and scooping and....MATCH THE HATCH.

Good luck on your new "this is how I spend my disposable income" quest. I got "diseased" about 55 or 56 years ago......still use some of the very first ones (pattern-wise) my great uncle tied for me, back in the Ozarks and they get me fish in Oregon.
Buy TWO boxes, one smallish, you can easily use/carry and one much larger for that ever-growing collection...big fan of split foam type.
PM me if you like, I might have fished water up your end of the state that you are headed to.

Not sure what the Cabelas kit contains but I would suggest you steer towards a 5 weight 9’ rod for NW fishing. I started out with an 8 weight 8.5’ rod back in the 70s and I caught fish but it’s a bit on the heavy side for our trout. I now have a 6 weight 9’ rod I bought off a buddy years ago and it works better but I’d love if it was a 5 weight. I don’t recommend you get too fancy on price until you feel your beginner outfit is holding you back. You can get poor buying more fishing skill. Kinda like golf. You can’t buy a better game.