Welcome new Stanger!
I am with you and just bought my 94 GT vert. I think the first thing I plan to do is a tune-up. New cap, wires, plugs, and an oil change to some Mobile 1 synthetic and a
K&N oil filter. At which time I will probably install a cold air intake. I don't like the clutter in the engine bay so I want to start by moving things around. A battery relocation kit may be next, then get rid of the smog pump, and possibly the A/C depending on how much it is going to cost to get it working again.
Looks like you already gave it a bath. I would start detailing everything as best you can. There are lots and lots of things you can buy for the 5.0's. But for starters get it cleaned up. Your car, like mine, is nearing 20 years old. I am sure there are things that are worn out and needing replaced. Anything that just needs some cleaning and repair get to it. Hoses, wires, serpentine belt, look it all over. It doesn't matter if it needs a new paint job or interior. Get it clean first. Get out a toothbrush and go to town. Trust me when I say doing so will help you get to know your car. Because you will be looking at everything you clean a lot closer. You will notice little things that need attention. Loose wires, cracked hoses, etc. And in the end you will be much happier with the car looking nicer.
As far as mods, Underdrive pullies, cold air intake, exhaust, switching to synthetics all make an improvement. Some more than others. Most of these can often improve gas mileage as well as performance. Gears are a real big improvement, but they are just the opposite. With a 5 speed 3.73's really stick you in the seat. But plan to spend a little more at the pump. It is up to you, your budget, and how far you want to go.
Appearance mods are great as well. This is all a matter of personal taste. Just know that the big wings and body kits are out of style. In fact, as far as the Mustang is concerned they never were in style. I am only saying this because I assume you are a kid

If your lights are faded or cracked replacing them will make a nice improvement. I am not real sure on all of the lights available, but I have yet to see an aftermarket light that is built with half the quality of a factory replacement.
My car already has an aftermarket cat-back which makes it nice to drive since it has a nice rumble to it. You might start there. If you have the factory cats they are probably due for removal or replacement depending on your emission laws. If your cats are plugged replacing or removing will make a noticeable improvement. Mustangs, in my opinion sound better than anything. And the best way to bring that sound out is with a cat-back. There are media files in the forums here. Listen to different setups before you buy.
Other than that it is all up to personal preference. I tinker a bit with my car every day. If you really love your Mustang you won't be able to help yourself.
Good luck, and again welcome. Great to see you younger guys starting off on the right hoof
