Hi, Migrator Soul, and welcome to DC!
I've lived here about fifteen years now, having also come from a place of flat, wide open spaces and a car-centric lifestyle (Phoenix suburbs), so I wanted to quickly send you my thoughts. First and foremost, DC is awesome and you're going to have a great time here, particularly as younger worker. The city does indeed feel very young and that's a great thing for someone in your position. Second, check out this page if you haven't already as I think it's a pretty good summary for newcomers:
https://www.reddit.com/r/washingtondc/wiki/movingNow then, off the top of my head here are some tidbits of advice:
- don't look to buy a place if you don't think you're going to be here for more than 5-8 years or whatever. Check out the rent vs. buy calculator at NYTimes.com as well as Michael Bluejay's calculator. I've been here fifteen years and my wife and I still rent (small condo in Adams Morgan). The upside is living right in the heart of the city in a neighborhood we love, and we like our neighbors really well, so it's a good situation. You learn to live in a smaller place and smaller quarters, and that will become a good thing. Think of it as mustachian training.
- don't get ahead of yourself and sell your car just yet. Try it out first to see if you'll really need it, and to get a feel for what it's like having a car in the city, street parking and all that. My guess is that you'll probably want to part with it after a year or two, but you can make that decision when it comes. That said, if you have a large car then maybe make this decision sooner as it's going to be a hassle. You'll quickly become an expert in quickly reading parking signs and regulations, knowing when and where you can park on what days and for how long, and of course in parallel parking. Things are tight.
- along those lines, I strongly recommend getting an annual (currently $85) Capital Bikeshare membership. You'll get a small key fob, and with that you've got easy and convenient one-way bike access damn near everywhere in the city. Download an app like 'Spotcycle' to see constantly updated bike docks. Get a bike helmet and get comfortable riding amongst the cars during the day. Bikers and walkers in this city are kings while those in traffic often are just poking along. Embrace it! Lots of locals, including me and my wife, use the Bikeshare bikes all the time. I much prefer to just ride or walk than ever go near my car unless the distance really calls for it or the weather is bad.
- don't be afraid of the metro. Yes, it's got a lot of problems no thanks to deferred maintenance going back decades, but to have a subway system in your city is still awesome. I'm lucky enough to ride a reverse commute out to the suburbs where I work, so I do avoid the rush in and out of the city each day. Yes, delays and sometimes breakdowns (trains getting offloaded, then you wait for the next one or whatever) happen, but in the big scheme of things it's still way better than the stories I hear from my colleagues with their car commutes from even farther suburbs. I read, play games on my phone, listen to podcasts, snooze, whatever. I love it and wouldn't trade it back.
- do try and live right in the heart of the city. I strongly urge you to avoid living anywhere outside the beltway because of a little bit cheaper rent. That's fine for some people and I'm not putting it down, but you should live in the city while you're a young, up and coming professional. You just can't put a price on the difference in the quality of life when you walk or ride everywhere you need to go. Find a group house or roommate(s) (which is what I did for the first several years) to help control rent costs. Yes, in the end it's still going to be more expensive than the same situation in your previous city, but that's just the way it is. Being in the city is great, and over the last ten years or so it's gotten better and better in my opinion ... way more good beer places, more laid back restaurants, incredibly good coffee places, clubs, theaters, and on and on.
- control your costs by avoiding driving (very easy to do here) and cooking at home. If you don't cook, learn! It's an essential mustachian skill anyway, right? Check out the many great farmers markets in the city, a few of which are even year round. Eat out with folks enough to be social and network as much as it necessary, but watch out for things like Sunday brunches and stuff like that. Yeah, it's fun but that stuff adds up really fast. Do you really need a bottomless champagne brunch every week? Enjoy yourself, but don't feel compelled to eat out all the time. You'll have a lot of colleagues who are enjoying their first big professional paychecks, and this is the moment where you make the smart decisions and really start building that 'stache.
- enjoy having four seasons! I'm from AZ, you're from TX, so I assume we're coming from kind of the same thing. Having a true autumn and spring season is something I don't want to ever give up, even once I one day move from DC to somewhere smaller. I've really come to love this part of living here.
- don't sweat it if people here seem colder or rushed and all that stuff. The area I was from felt friendlier by comparison, and I took the change hard when I first came here. There's a kind of rhythm or city speed to things, and you'll soon get into it and enjoy being part of it. It's not as hectic as New York, but you'll notice the difference.
- you mentioned being introverted, and I am, too. Believe it or not this is a good place for that, too, since there are lots of places to park yourself like great coffee places, parks and all that, or you can just go for an aimless walk. With the architecture and older buildings, there's always some new detail you didn't notice before. My place is pretty small, and I often forget that that alone is a good excuse to just go outside and start walking when I'm feeling bored or getting that consumer itch to start looking at bigger places. It's a good reminder that I live where I live for the location. I walk out my door and I'm right along a main street, with bars, shops, a park just two blocks away, coffee, a bookstore, a library, whatever. I think it'll work for you.
I might think of some more, but enjoy this short list for now. Most of all, enjoy it and have fun. Even after all this time I've been really glad I moved here. The first year was a tough transition because I was kind of resisting the new culture, always comparing it to where I came from, but I eventually just had to get over it. Now I can't remember what my problem even was.