|If it fits in the closet, it's put away.|
Rather than tell you how you should plan a trip, I'll just relate what I do, and you can decide for yourself whether I'm on point.
I'm in the PNW, so there's plenty of areas to backpack, and most of them involve rain, especially in shoulder seasons. In terms of choosing a hike, I usually start with the Washington Trails Association (wta.org). They have the most comprehensive guide to trails in Washington, and plenty of user trip reports to help you choose a hike. I'll google the hike for other reviews, or see if its been mentioned on reddit or backpackinglight (see links on sidebar). Once I've chosen a spot, I'll note the reservation requirements and whether I need a state or federal recreation pass to park at the trailhead.
Most backcountry camping trips require a wilderness permit. Permit systems allow parks to manage the number of people in a backcountry locaiton at any given time. It woudn't be backpacking if the trail had a rush hour. For the most popular hiking trails, especially in National Parks, there are often lotteries/limits (Wonderland Trail, John Muir Trail, etc.) for permits. It helps to be flexible and have a backup plan that doesn't require a lottery. Also check whether the park holds any permits for walk-up visitors. In Yosemite, for instance, permits for some areas are only 60% reservable in advance, and the other 40% they keep available for walk-up reservations at the ranger stations in the park. Whichever you choose (reserve in advance or walk-up), have a backup plan.
The Packing List
This is where I can enterain myself for hours. I can evaluate every piece of gear, clothing, and food I'm bringing and decide whether it is worth its weight to bring, how much to bring, how to pack it, etc. Especially since I've started to get into lighweight backpacking, my gear lists get more and more detailed. If you don't have a digital scale to weight all your gear, you're definitely missing out on the fun.
Usually I'm packing for two people (my wife and I). My wife is interested in backpacking with me, but I don't think she'd be a backpacker without me. This is fine by me, since I enjoy the planing and packing process.
Below I've linked to a generic gear list for an average 3-season backpacking trip. I wouldn't call this list ultralight, but I do think it is on its way. If you're new to backpacking, there's no harm in bringing a few luxuries (pillow, collapsible water basin), but if you're looking to minimize weight, there's plenty of ways. So long as you're bringing enough to be safe in the wilderness, you can define what "comfort" means on your own. If I'm by myself (or at least packing for myself) I try to keep it simpler, sticking with no-cook foods so I don't need to bring a stove, for example. When with my wife, I add in some luxuries, cook hot meals/drinks, and make sure we'll stay warm and dry all day long.
My list is always a work in progress, and links are just examples: Google Spreadsheet
Leave early; traffic sucks. Also, stock your car with a couple things that you'll want post-trip, like a few cans of water/soda/beer and a few snacks. Watch out for critters, though. If you're in bear (or mouse) country, there's some risk of leaving smelly foods in the car. I've done fine with sealed drinks, but usually skip the snacks unless there's a trailhead bear box available. A change of clothes and a box of wet wipes/bathing wipes can be nice if you're planning on running into town for a burger (which you should). Some people might consider the change of clothes unnecessary, but I like to clean up a little before driving home.
I enjoy planning a trip, planning gear selection, and then testing whether my plans were successful. It adds a whole second dimension to the trips beyond the enjoyment of the outdoors. I'm trending toward lighter gear, but basecamp style backpacking is also popular. You could carry everything but the kitchen sink for a few miles into the woods, setting up a basecamp, and then spending a couple days exploring the area during the day and having a fully stocked camp at night. In either case, the gear closet is part of the fun.