This is me responding as a fellow traveling human - not in my role as webmaster.
"this will be my first plane ride ever and I will be alone...I honestly have no idea how to handle the airport"
G3rry had a lot of really good advice, and I'm thinking we may also need to cover some of the basics of using an airport if you have zero experience. I also know Seatac well.
Using the SAS app, check in for your flight 24 hours ahead of time. Check the app again before you leave for the airport to see if there are any delays to your flight.
Get to SeaTac 2.5-3 hours before your flight leaves (many do 2 hours or even 1.5 for international, but this is your first time... give yourself time to navigate the airport). When you get to SeaTac, you'll presumably get dropped off by family or a friend at the Departures level. As you enter the Departures area by car, there will be big signs above each doorway to indicate which airlines are served by that area of the airport. Find the sign that says SAS and get out of the car there. (If traffic is insane at the Departures level which can happen, sometimes the readerboards on the way into the airport tell you to go ahead and use the Arrivals level. It's the same situation there... just take the stairs up to the Departures level once you enter the building). If you're arriving via light rail, it's even more straight forward. Just get off at the Seatac stop and follow the signs. You'll be at the Departures level.
Once you enter the building, look for the kiosks with the SAS logo above them, preferably the kiosks with conveyor belts behind them. You'll wait in line, talk to the human at the kiosk, show your ticket (SAS app) and probably your passport, and then they'll take your bag and off it will go on the conveyor belt. You'll pick that up again in Rome (in Rome, follow the signs when you get off the plane to Baggage Claim - or the Baggage icon since it may be in Italian ;).
After you've checked your bag, it's time to go through security. Check your SAS app to see what gate your plane is boarding at. It will include a letter and a number. Like B23. B is the section of the airport and 23 is the gate number. If your plane is departing from the A,B,C or D gates, you can pretty much choose any security gate, but the main one in the middle (you'll see) is likely where you'll go. Nope, you can't use any of the pre-check or Clear lines that look shorter unless you already know what that is. For the N gate, it's usually best to use the security line open at the north end (far right, you'll see), and for S gates it's usually better to head to a security line near the south (far left).
Get in line... this is where most of the 2.5-3 hours is spent. Sometimes it takes 5-15 minutes, and I've waited over an hour before. Frequently there are people at the head of the lines that may say "go to X security line that way to get through faster." Sure, go ahead if it saves you time. At the head of the line, you'll show your ticket (in your app) and passport, then you'll go through screening. Just follow the directions they give you and take a look at how others are doing it. As a reminder, they won't allow any containers of liquids over 3oz. They'll make you throw out anything with more. If you bring a water bottle, bring it empty, then fill it after security.
After security, head to your gate, and it's as straight forward as following the signs. You're looking for the lettered gate areas first, then the gate number. If you're going to the N gate, you'll follow signs to a little underground train that takes you there. For S gates, it's the same, but this is also undergoing transition. I believe you can also walk to the end of the A gate and take the new bridge that takes you up and over to the S gate. Again, just follow the signs and you'll get there.
Once you're at your gate, wait until boarding begins (usually ~45 min before departure). More on boarding...