Welcome to the forums! We'll need to know a bit more about your car to get you goin in the right direction. For a 1998, you could have either an AHU engine, or an ALH. What car do you have? Golf, jetta, New beetle?
Also, just as an FYI, these diesels don't have a throttle valve, and it's been proven over and over again the stock intake ducting is more sufficient than cold air intake on them.
By popular demand, here's an FAQ thread for USEFUL information on cold-air intakes and high-flow air filters. Useless posts will be removed. Go nuts. Edit: know what would be cool? Maybe someone can make a nifty, tastefully funny but small and useful button picture that links to this thread...
Also you have no throttle body.
You have a long way to go to not sound like a 16yo troll from back in the 2000s.
You have an extremely old engine that has a lot less support for performance now days.
Anything you do other than a light 1.5 tune is going to now or eventually blow the clutch.
Change the clutch.
Some light upgraded nozzles. 502s are good, bigger turbo, and a tune and a few bits here and there, pd130 or 150 manifold for instance. You have a lot of reading to do. Plan in 3 or 4 grand to do anything that will compare with a modern power level.
What you need to do is get all the fluids, filters, rubber bits (belts hoses) and timing belt up to date and the car running in proper order, refresh the suspentio. And find and fix your rust. You have it. They all do. All of them. From the factory! No seriously!
Good news is your car left the factory with a cold air intake, same as every other car since about 1980 did. It also runs at wide open throttle all the time, and is limited by fueling and that teeny weeny turbo. The inlet is about the size of a silver dollar, you cannot force-feed it. Unless you have another turbo....