Okay so when it comes to modifying your car the cheapest and quickest way to get some extra power can be getting a new
aftermarket intake. Typically prices don't cost over 400 dollars and the installation doesn't take more than 10-15 minutes. In most cases atleast..
When it comes time to pick out what intake is right for your car it may not be as simple as you'd think. You can usually pick from either a cold air intake or a short ram air intake. The cold air intake usually uses a longer intake pipe to place the air filter in a place it can suck in the most air. (Moar air = moar pweR!)

See that blue pipe? That's the intake. It's heading down into the front bumper area to suck up that delicious air. nom nom.
And if you're a real baller you'd pop out your headlight and use that for your cold air intake.

Heh heh, he's winking at me.
If you don't feel comfortable driving around knowing that your cars air intake is just potentially a few inches above the ground then there are other options. (pro tip: Don't drive over deep puddles like a jackass when you have a cold air intake like that. If you suck up a nice few inches of water then you better start looking for a new engine...ass)
You can go with a short ram intake. They
usually use a shorter intake pipe and sit where your factory oem intake and filter box went.

See how much shorter a short ram intake is? I don't lie to ya.
There's still a lot of debate as to which intake system is better. Some people say the ram intakes sit in a bad spot and just suck in warm air which doesn't really do much of anything. Common argument is that when you're driving along there's going to be cold air coming into the engine bay so if you're doing a dyno test you need a big ole' fan blowing at your car while you do it. (discuss in comments if you'd like?)
My opinion: Get a cold air intake. They're harder to install but you're guaranteed results. Usually around 10 hp or so switching from a stock intake. And in the mazda 6 (I think it's the mazda 6, idk I'm not a mazda pro) With an AEM cold air intake it gained 20 hp. That's amazing for a 300 dollar part.
Okay so you've decided to buy a CAI (cold air intake)
Now you just gotta pick with company you want to buy it from.
The biggest names when it comes to cai's are AEM, Injen, K&N, Mugen (hondas), and Fujita. But there are a bunch of other ones like option racing, password jdm, stillen and weapon R just to name a few.
I actually really like Weapon R intakes. I'm not sure how good they are but damn do they look cool haha.

This one's called the dragon. Ha, Just imagine opening your hood and seeing that just staring back at you. It makes me feel like a little schoolgirl again.
Or incredibly dorky saying "Yeah, I have a dragon under my hood. What do you have? ...chump"
But for serious business. I gotta recommend AEM intakes. They're probably the best value for money and they will certainly give you decent power gains. My only real complaint with AEM is that they just always play it safe. They're so boring. Typically when you buy an AEM intake you get an option of like...2 or 3 colors. And then if you're lucky you get a snazzy license plate frame for their free advertising. That's why I like Weapon R, they do weird shit, I like weird shit.
However, If you're just looking to replace an air filter then I say go with a K&N drop in filter. Cheap yet effective and they come with a million mile warranty.
Just don't expect any real noticeable gains of power.
I hope this somewhat served as a crash course, mainly for people who don't know a whole lot about cars or car modding. If you have any other questions either leave a comment here on this blog or send me a message and I'll try to help you out best I can!
And if you're wondering, No I'm not getting paid to write this article. I wish I was D: Hey auto companies, Give me free stuff and I'll write good reviews
*Wink wink*
I'm waiting on you, Nissan. Let me drive a GTR Spec V!

Oh lord. Time to change my pants.
I think there just might be a Skyline article coming soon...