Welcome to StangNet, and congrats on the purchase.
As far as starting mods, I would suggest you think about some mods other than power (as suggested by others). For example, since you have a 'vert my suggestion for a first mod would be subframe connectors. They will noticeably stiffen the car. You don't need to do this first, but keep it in mind because you will probably want to do it before long.
Otherwise, exhaust is a good early mod. It won't add much in terms of power, but the improved sound will convince your ear that the car is faster.
Hint on exhaust, mid pipes are for volume (off road is considerably louder) and cat-backs are for tone. After that, the choices between mid-pipe styles and cat-back brands are primarily a question of preference.
Definitely do the free/cheap mods: bump timing up, remove air silencer, install K&N.