Don't modify a Matrix. Just drive it and enjoy it for what it is. 

If you try to mod it, you will come to hate it.

Learn the ins and outs about cars, then work on modifications. In my opinion, you need to have a decent understanding on how engines work, as well as other components on your car like the drivetrain, suspension, steering, brakes, etc. Then, in turn, you will have a good understanding on modifications. Because that's what the modifications are for...to improve the performance of those components, and you need to understand how it will improve them. 

What's the benefit of a cold air intake (cold air -> denser air -> more oxygen -> more fuel -> more power)? What's the advantage of a free-flowing exhaust (scavenging effect)? Do bigger brakes really provide more stopping power (friction doesn't care about surface area...bigger brakes are more for heat dissipation)? And so many more questions...Stuff like this is worth learning to know what the modifications do to your car, and whether you really need them for your application OR whether a TUNE is required (what's a tune?...another question to ponder).

It's not just about bolting on stuff that "looks aftermarket/cool" and calling it a day. There are so many people out there that just start modifying their cars without any knowledge on anything, and then they ask why their engine blew up (someone the other day asked on this website why their engine blew up lol...they got greedy with turbo boost).

Enjoy the car you have now without modifying it (and even do some at-home maintenance on it as a start), learn a few things about engines/cars in general on your free time, and then when you get a nicer car later in the future you'll be able to modify it in a smart way. 

Good luck!

body kit, spoiler, loud exhaust and fake tips, stuffed toys in the back window

I kid. Be happy that you have a working car.

Have a little fun if you want, but since you’re young and it’s likely your first car, don’t put too much money into it. That’s an 18 year old Toyota, but it might run another 10-15 years. If it’s ultra high mileage and poorly maintained, might only last a few years. So two things: if you plan on driving that thing forever, think ahead into the future and make sure there’s nothing dumb and irreversible on there that you might regret when you’re in your mid- or late-20s. Now if it’s just a stop-gap car and you’re planning on upgrading, say, after college, or say it’s got really high mileage and seems like it might go out in a few years, you obviously don’t want to put too much money into mods because you’ll regret spending that much when it goes out or you want to sell it. Remember, mods are to each his own, so when you go to sell it, no one will care about your mods, and if they know what they’re doing they won’t pay any more than what anyone else is paying for a stock one, unless it’s been totally overhauled into a full on race car or something. The mods will not add much value to your car, and possibly it might even decrease its value. As to my advice: it’s a Toyota Matrix. It’s not a sports car or anything fancy. So I would say just keep it stock and have fun driving it around with the manual.

I would not recommend any mods for teens. 

Governor

Can I interest you in some stickers? Each sticker usually adds ~5bhp.