Hi Jessica, and welcome to MW.
The Mustang 5.0 responds well to modifications.
The first and best mod you can do for the '89 is to install a 3.73 rear (stock is 2.73 or optional 3.08).
A K&N air filter is a natural first step, as well as a full tune up. Timing can usually be advanced to 14 degrees (initial) with no problems (and snappier throttle response) as long as you run 92 octane (or better) gas.
An aftermarket shifter like a PRO 5.0 is also a 'must have' to get the most out of those quick-climbing 5.0 revs.
Sub-frame connectors are also a very wise investment, especially on an older 'Stang like yours. Subs noticably improve launches and handling and tighten up the whole body.
Exhaust is a favorite area to play with and there are a ton of muffler choices but an H-pipe should be the first exhaust mod as the 4 catalytic converters are the biggest exhaust bottleneck.
The factory headers are O.K. with stock heads but if you plan on installing aftermarket heads, you'll naturally want better headers. Lots of choices.
Just a few basic mod suggestions to get you started.
Good luck with your Mustang - and come back often.
