98 taurus air conditioner

I would like to fill my air conditioner, but I can't find where I should fill it up.

you don't fill it up, you charge it, I recommend you buy a book detailing how to do this if you intend on doing it yourself

This is a fairly involved process that is only environmentally friendly if you have some specialty equipment. I think that for most people this is a job best left to the professionals.

Why do you need to "fill it up"? If there is no refrigerant in it, there is a leak somewhere. This needs to be repaired or you will be spending a lot of time "filling up" the system over and over. If you know nothing about A/C work, think twice before you do anything to the system. Good luck.