Congrats and welcome! One of the moderators will be along in a bit and remind you to fill out your profile.
There are a lot of mods you can do - depending on where you want to go... remember to use the SEARCH COMMUNITY function for the site - it will help you find a LOT of information. Use ADVANCED SEARCH for more detailed requests.
I purchased my 2017 XT premium about 6 months ago and started with LED turn signals and back up lights as my first mod. It was an easy upgrade has a high bang for the buck factor. Bought everything for the mods on Amazon.
For turn signals, you’ll also need to buy a flasher relay for LED bulbs. The cheap analog ones are around $9 (which I bought), or you can get an upgraded relay (like the one from Diode Dynamics) for around $50 that gives you more signal options. The back up lights are plug and play. Plenty of install instructions here if you search the forums.
You're going to confuse some people with a "2019 Forester XT" in your profile.
I did LEDs everywhere, including the interior, as @alexdub2 did with LEDs sourced from Amazon. The brightness and whiteness of the LEDs on the dome lights is a really big improvement, and with some careful searching (and some trial and error) you can find good LEDs for everything on the exterior. I even did my backup lights. I also changed the rear turn signals to red LEDs, instead of the amber. I just think they look better red. I'll probably do the Diode Dynamics flasher relay at some point, too.
20mm Subaru OEM rear sway bar as @adc suggested. Or if you want an adjustable sway bar, I've heard good things about the Whiteline 20mm. Get new end links while you're at it, because you're really likely to strip out the Allen key on the end of the end link when you try to remove them.
Catback exhausts can make a nicer sound, if you're looking for it. I replaced the midpipe with an STI midpipe. STI owners are always selling them cheap, it seems. And I put Nameless 5" mufflers on. Not much highway drone, but it makes a little noise if I get on the throttle.
Obligatory cold start video:
Some other little upgrades:
Third brake light strobe:
LEDs to replace the bumper reflectors, sequential turn, sequential 4-way, and brake lights: