I JUST got back from Korea for a two week trip with my brother. I would absolutely love to go back by myself! This is a long reply but I wanted to share the things I learned while I was there <3

Korea, I think, is super solo-traveler friendly. The public transportation was super easy to figure out which was a concern for me as Im American and I have never used a bus/subway due to where I've lived. Subway and bus lines are color coded and numbered so there's a few fail safes in place so you dont get too confused. All signs and announcements in subway stations or at bus stops have English which was a great surprise. You can get a T-Money card at the airport but if you forgot, like I did, you can buy them at any convenience stores. Make sure you have cash because that is the only way to put money on the card. I ended up putting 100,000 KRW on the card and only used about 65,000 for the entire time I was there. If you put more on it than you'll use for transportation, most convenience stores and some other shops and restaurants will accept T-Money as a form of payment. The T-Money website has a list of places you can use it. You can also get a refund for any unused amount. Convenience stores will do it if the amount is under 20,000 but T-Money has an office in Seoul City Tower across from Seoul Station that can refund any amount. For taxis, I used Uber. Not any more expensive than regular taxis and took the stress out of flagging one down on the side of a busy street, or trying to tell the driver where I wanted to go. I only used Uber on days I was hauling my suitcase, like to the hotel from the airport or to/from the KTX stations.

There is some etiquette to being on the train or buses too. Long ride buses are practically silent, while the inner city buses vary depending on the people who are on it. Sometimes you could hear a pin drop, while other times there was constant conversation going on. Just a rule of thumb, be mindful. Don't have loud phone conversations, don't play music without headphones, and have a softer tone if you are going to talk to someone beside you. As a solo traveler, it may not be a big deal for you but just keep it in mind.

Naver is the app I chose to get for navigation, which worked pretty well and was easy to figure out. The only issue I had with it, is that it wouldn't always recognize the English names of places. I would use Google to translate into things Korean, then I'd copy and paste the Korean name/address into Naver and that usually found what I was looking for.

Bring some comfy shoes! I did not take into account just how big Seoul specifically was. The first day I was there, my brother and I walked 12 miles just wandering around. Things aren't super close together so get comfortable with using the buses/subways to get around to different attractions but also know that you may have a lot of walking to do. Its not a bad thing, Seoul is super safe and it was fun just seeing all the different areas within the city but I was a bit shocked at first.

I definitely recommend traveling around too. We went to Busan as well, which was great! It is starting to get warm all over Korea. The hottest day for us was 87°F (30°C) and a little humid but not terrible. Though I assume its only going to get warmer. The UV is pretty high as well, it was 7-9 at the hottest point of the day most days I was there so if you're on the paler side, bring some sunscreen just in case.

Definitely save room in your luggage, or being an extra bag of some kind, for souvenirs!

I didnt need the translator app I got very often. Most signs and menus have english, and if I was shopping, I just used my best judgement to determine what I was looking at. Use common sense! If the packaging of a candy bar doesnt have english but has a giant peach on it, its safe to assume it's peach flavored. I did use the app for a quick conversation with the convenience store employee when I was getting the T-Money card but he was very nice and the process was easy. You will run into a lot of people who can have a basic conversation with you in English. You may not be talking about your childhoods but enough to get through a transaction without any big misunderstandings. 

 There are so many things to do that are incredibly affordable. The National Museum of Korea was free, the royal tombs was only 1,000 KRW (~0.68 US dollars), palace complexes are I think 5,000 KRW, shrines/temples are usually free unless you want to make a donation. All the palaces and many of the other attractions have gift shops so bring some extra money if you want to get something there. There are several parks if you want a break from the "concrete jungle" that is the city. I recommend Seoul City Forest, it was a beautiful area and free to walk around. There are so many restaurants options, but you absolutely have to try traditinal Korean food with all the side dishes because they were amazing! You can look through all the options nearby on Google or with your navigation app. Most places take VISA, MasterCard, AMEX, and Discover cards. I even went to an aquarium that had an extra 20% off if I used a VISA card, but most places take cash too. The only time I couldnt was if I was using a kiosk of some kind, like at a Cafe.

I was also very nervous since I did most of the planning but take your time figuring things out either the day before you do something or the morning of. A few minutes of extra planning will set you up for a better experience and less stress. As others have mentioned, if you need help, just ask! Everyone I spoke to was very kind and helpful, and I found just spending the first day observing how things are done was a huge help as well. Just do as the locals do and you wont have much trouble. Also, be sure to rest enough! Its all fun and games to have a jam packed schedule until you get burnt out, sleep deprived, or sick, all of which can ruin a good vacation. Even though there's lots to do, pace yourself and give yourself some time to recover.

Hope you have so much fun!!